These challenges can be obtained in either single player or multiplayer.

1) FIELD MODIFIED
"Purchase a marine weapon upgrade."

Once you reach rank 2 -which should take no time at all -you will be able to purchase one upgrade from the arsenal (pause the game, the arsenal upgrade option is in the menu). Just pick anything and buy it to unlock this.

2) SPRAY N' PRAY
"Kill 5 enemies while hip firing."

What this means is that you need to kill 5 enemies without looking down the sights of a gun. Given how brain-dead the A.I. is, this is a really easy challenge.

3) TWO FOR ONE
"Kill two enemies with one shot from an Underslung Grenade Launcher."

What you need to have here is a weapon with a grenade launcher attachment (such as the pulse rifle) and a group of aliens/humans. Shoot at the group to weaken them, then fire a grenade to kill them and unlock this challenge.

4) BOOM STICK
"Kill 20 enemies using the Pump Shotgun's primary fire."

Easy, just let enemies get close to you then fire away to kill them quickly. Just make sure you have the pump shotgun -kills with the tactical shotgun or Hick's shotgun do not count towards this challenge.

"You purchased a game with the SEGA logo on the box and thought it would be good? Silly child."

5) BIG FUCKIN' SIGNAL
"Track 5 xenos at once on your Motion Tracker."

There are several points in the game where you'll have to battle large groups of aliens in small spaces, just briefly use your motion tracker to unlock this. Big battles can be found in "Hope in Hadley's", or you can try online at the start of a game -I've seen it happen before where all 5 of the aliens players rush forward together.

6) HEAVILY ARMED
"Kill 7 enemies using a primary weapon's alternate fire."

Have the pulse rifle in your primary weapon slot, then use the grenade launcher on it and go and kill some enemies. If you run out of ammo, go into the arsenal menu and switch to either of the pulse rifle's other alternate fire options, or swap in a different weapon into the primary weapon slot.

7) KILL OR BE KILLED
"Kill 20 enemies with any rifle's primary fire."

Not much to say about this one, another very easy challenge. I got it with the pulse rifle.

8) STRAIGHT SHOOTER
"Get 10 headshots using the Service Pistol."

This can be obtained by shooting humans or aliens, but I found it easier shooting humans. For this challenge you'll most likely want a couple of upgrades for this gun, such as laser targeting and the A27 high velocity rounds. Aim down the sights and obviously go for the head before pulling the trigger, this should take out a human enemy with one shot on the easier difficulties.

9) AIRTIME
"Kill 20 enemies that are climbing on the walls or ceiling."

Probably the fastest way to get this one is by going into the "No Hope in Hadley's" mission and getting to the part where you have to fight some aliens in a long sewer tunnel. You should have access to a smart gun here, and since it has auto-tracking it makes shooting the aliens a piece of cake. The aliens are often running across the walls or ceiling in this section.

10) POP GOES THE MELON
"Get 10 headshots on xenos with any rifle."

The assault or battle rifles are what you'll want to use for this one. If you see an alien at a distance which hasn't spotted you, use the battle rifle's zoom scope to get an accurate shot at the head. Otherwise let an alien get close to you, aim up, and then fire.

11) SHORT, CONTROLLED BURSTS
"Do not miss an enemy with any bullets from a full rifle mag."

Use the battle rifle for this as it has the smallest magazine size of all the rifles (15 bullets). What you need to do here is let an enemy get as close as possible, and then fire. Then repeat this until all the magazine is empty. However an alternate option is to play through the mission "Home" as the entire level is a fight against the alien queen, who is a massive target which is hard to miss.

12) SHARPSHOOTER
"Kill 20 enemies that are 15m away or father, using the Battle Rifle."

The battle rifle is basically the game's equivalent of a sniper rifle, and is fairly powerful also. There are lots of points throughout the game where you can pick off enemies from a great distance away, so try and aim for the head to take them down quickly.

13) TRAPPER
"Kill 20 enemies with Battle Rifle Mines."

The battle rifle can have mines added as an alternate fire option, but it can only hold four of them at a time. You can shoot mines at walls and they will stick, and will then detonate when an enemy gets close. But a much easier way is to just fire mines directly at enemies, as they'll explode on contact. You can kill an enemy with just one mine, but sometimes it takes two. As ammo for the mines seems to be sparse, you'll want to reload checkpoints a lot for this challenge.

14) SECOND TO NONE
"Kill 15 enemies with any sidearm."

For this challenge you need to use any of the following weapons: service pistol, combat pistol, Vasquez's pistol or Gorman's pistol. Not that difficult to do on the lower difficulties, although you might want to do this one against human enemies, as they take less bullets to kill.

15) CARRY A BIG STICK
"Kill 3 enemies with a primary weapon's alternate fire without dying."

Probably the best way to do this one is to use the pulse rifle's grenade launcher, as it can take down groups of enemies easily. Just don't be that close before firing.

"When people consider this to be the highlight of your game, you've probably done a bad job."

16) BACK ON YOUR FEET
"Revive 5 other players in co-op Campaign or Versus matches."

If a player is downed, a message will come up on screen stating this and for you to go and help them. To revive a downed player, you need to be near them, then press and hold the relevant button (X for the Xbox 360 version, but a message will appear on screen telling you want to press) until they are back up on their feet.

In versus, you can revive other players in the survivor or escape modes. But this may be tricky as the players on the alien team are obviously going to try and kill you when you are trying to revive another player. Downed players can also die easily (they have to be revived within a few seconds, otherwise they die).

In campaign mode, your best option would be to play some online co-op in random games on the ultimate badass difficulty, since players go down a lot easier, and there is also friendly fire. Another option for the console versions would be to play split-screen co-op and have your buddy deliberately down themselves (such as blowing themselves up) so you can revive them.

17) EQUAL OPPORTUNITY
"Kill 20 enemies using the Combat Pistol."

The combat pistol is a sidearm weapon, try using it against human enemies as they die faster than the aliens.

18) SET 'EM UP, KNOCK 'EM DOWN
"Kill 10 enemies with Claymores, Mine Launcher Mines, or Acid Traps."

Claymores are tactical weapons, which you can only ever carry one of. Acid traps are an attack that only the xeno spitters can use, meaning you will have to play some games online. So the easiest and fastest way of doing this one is to use the mines, which are an alternate fire option for the battle rifle. The best way of doing it is to fire directly at an enemy, as they'll explode on contact. The battle rifle can only hold four mines at a time, so you'll have to reload the checkpoint a few times.

19) SPREAD THE LOVE
"Kill 2 enemies within 10 seconds using the SMG."

The SMG is kind of like a machinegun, which fires fast but doesn't do much damage. Killing 2 enemies within 10 seconds is easy through, but again, this challenge is easier to do against human enemies rather than the aliens.

20) XENOPHOBIA
"Kill 10 Soldier, 10 Lurker or 10 Spitter xenos."

This looks and sound like a multiplayer challenge, but I got it in single player -the mission I played was "Hope in Hadley's", and it unlocked just after the first door opens (where you have to defend the door and have access to a sentry turret). Strangely I killed a mixture of aliens -about 7 soldiers and a few spitters -but this still unlocked anyway.

21) BLOWN TO HELL
"Kill 20 enemies using the Assault Rifle's Grenade Launcher."

Easy, just aim and fire. The grenade launcher does big damage and can often kill an enemy in one hit, two at the most. The assault rifle can only hold a few grenades, so reload the checkpoint if you run out of ammo.

22) MAKE IT COUNT
"Shoot enemies with an entire clip using the Service Pistol, without missing or reloading."

The service pistol is a sidearm weapon, and a clip for it holds 12 bullets. You can try this against normal enemies, let them get really close to you and then fire a couple of times. Otherwise playthrough the mission "Hope in Hadley's". After you complete the part in the vehicle bay, you'll have to go outside and fight a crusher. The crusher is a large alien, which will charge at you -if you avoid being hit by this the charger will crash into a wall and take a few seconds to turn around, so start shooting and then repeat this process. Another way is to play "Home" as you have to fight an alien queen, which is a massive target.

23) DOWN BUT NOT OUT
"Kill an enemy while incapacitated."

What this means is that you need to kill an enemy whilst downed. You will have to get this challenge in co-op or versus (in the survivor or escape modes), but playing co-op is the easier option. When you are downed, you can only use a sidearm. I got this by playing co-op on "Battle for the Sulaco". When the mission starts, run towards the door where the first enemies appear, and blow yourself up with something like the pulse rifle's grenade launcher. Then as soon as the enemies enter, shoot at one of them. The weapon I used was the combat pistol, which I'd upgraded with the extended magazine and full-auto action replacement upgrades.

24) AMATEUR SURGEON
"Decapitate a xenomorph."

This requires you to blow the head off an alien. I unlocked this one by using the pump shotgun, although I imagine there are plenty of weapons that will let you do this.

 

"If this was a real game, it'd be better than Aliens: Colonial Marines."

 

25) FLASH FIRE
"Kill 2 enemies with fire damage within 10 seconds."

There are several weapons you can use to get this -some missions have flamethrowers in them, but you can also use the pulse rifle's firebombs (an alternate fire option which you must purchase). The assault rifle also has the "incineration unit" alternate fire. If you got the limited edition version of the game, you can also use Ripley's flamethrower. The flamethrowers can burn up any enemy quickly, killing two of them within 10 seconds is easy.

26) JUDGE, JURY, AND EXECUTIONER
"Execute 20 enemies using the Pulse Rifle's primary fire."

I guess they were running out of ideas for challenges at this point. Just kill 20 enemies with the pulse rifle's bullets.

27) SHOTTY WORKMANSHIP
"Kill 20 enemies using the Tactical Shotgun's primary fire."

Another boring challenge. The tactical shotgun works best a close range, so don't shoot at enemies far away from you with it.

28) KEEP UP THE FIRE
"Kill 6 enemies in under 60 seconds using the SMG."

If you managed to kill 2 enemies in 10 seconds with the SMG I'm sure you can kill 6 enemies in 60 seconds with it. You may want to buy some upgrades for it, such as the extended magazine, to make this a little easier.

29) REFLEX ACTION
"Kill 5 xenomorphs that are inside a vent using the Pump Shotgun."

If you intend to boost this, get your buddy to enter a vent in any multiplayer map, then kill him with the pump shotgun and repeat this four more times. However, I got this in the "No Hope in Hadley's" mission. After the aliens first appear, you and O'Neil have to make your way back towards the command centre. There is a part where you will have to go down a corridor, and as you do an alien will appear out of the vent to the left -kill this alien and it'll count towards the challenge. Reload the checkpoint and repeat this four more times.

30) THE SILENT KILLER
"Kill 5 enemies using a Suppressor on the Battle Rifle."

The suppressor is an attachment upgrade for this weapon which you can purchase. All it does is make the rifle shoot more quietly, so beating this challenge shouldn't be a problem.

31) AMPUTATION
"Remove 20 limbs from xenos using the Tactical Shotgun."

This is an annoying challenge as blowing off the arms and legs of aliens with this weapon seems to be a pain. To make it a little easier, try stunning an alien with the shotgun's arc rounds (an alternate fire option) then quickly and carefully aim and fire at the legs or arms.

32) STAND YOUR GROUND
"Make the killing shot on a Crusher using the Service Pistol."

This is the most difficult challenge in this category. You can boost it by playing online, but if that isn't an option, playthrough the mission "Hope in Hadley's". After you complete the vehicle bay part, you'll go outside and a crusher will appear. The crusher is a large alien that will charge at you if you are a distance away from it, otherwise it'll stomp the ground if you are close. During this section you will be with the A.I. controlled O'Neil and Cruz. The crusher will ignore them, but they will fire on it. There are also several normal aliens, kill all of them first. Once the normal aliens are dead, you will want to fight the crusher in a place where O'Neil and Cruz cannot shoot at it (they will remain by the door when they reach it) so lure it away from them.

The crusher has a lot of health, so unload a few magazines from other weapons into it first (I used the phased plasma rifle) then switch to the service pistol. The crusher is immensely stupid, and you should be able to avoid it when it charges easily. After it charges, it will crash into a wall, and then take a few seconds to turn around before charging at you again. So fire away with the service pistol when it does this. This may take some time but as long as all of the lesser aliens are dead, and O'Neil and Cruz cannot help you, you'll get it eventually.

33) COMBINED ARMS
"Use the Combat Pistol to kill 10 enemies that were on fire."

This is easier to do against aliens, as they are more resistant to fire than the human enemies. Use the assault rifle's incineration unit to light up some aliens, then quickly switch to the combat pistol and start shooting. Upgrading the combat pistol so it has an extended magazine and the full-auto option helps.

34) FEEL THE BURN
"Kill 20 enemies using Firebombs."

Firebombs are a tactical weapon which will explode once thrown, creating a small fire which will burn up enemies. They're quite powerful, but will kill human enemies faster than the aliens. I think you should also be able to use the pulse rifle's firebombs alternate fire upgrade, although I can't confirm this.

35) ASSAULT WITH INTENT
"Kill 20 enemies using the Assault Rifle's primary fire."

Weeee, back to boring challenges. Just do what it says.

 

"In space no one can hear you scream for a refund."

 

36) LIGHT 'EM UP
"Kill 20 enemies using the Assault Rifle's Underslung Flamethrower."

The flamethrower is an alternate fire upgrade for the assault rifle you'll have to purchase. Once you've equipped it, all you need to do is kill 20 enemies with it. The flamethrower doesn't have that much ammo, so reload checkpoints to speed things up.

37) SHOCK TROOPER
"Kill 20 enemies that were stunned using the Tactical Shotgun's Arc Rounds."

The arc rounds are an alternate fire upgrade for the tactical shotgun. When they hit an enemy, the enemy will be stunned for a couple of seconds, so as soon as you stun someone, blast away to kill them. Do this 20 times to unlock this final challenge.