Aliens vs. Predator

Probably the easiest place to get this achievement if you don't want to bother playing the entire marine campaign before getting it is in a survival match on C Block. This method also allows you to not worry so much about ammo and holding onto a gun that makes you run so slow.

When you start a survival match on C Block, it starts with you locked in a corridor with a smartgun sitting right in front of you with a large stack of boxes beyond it. Grab the gun and start backing up but continue facing the obstruction. The first wave of drones will mostly come up over that obstruction along the ceiling. When you see several of them but before they actually reach you, hold the LT and they should start dying. Most likely, you won't be able to see 5 of them at one time before they start getting into attack range. This should be fine because usually several more will appear in the exact same place as the first few start dying. Whatever you do, remember not to let off the trigger until you are either out of ammo in the clip (at which point you're pretty much SOL and will need to try again) or 5 have died and you got your achievement. Letting off the trigger at any point resets the counter.

I found a pretty easy way to get it when your playing the marine missions. During the ruins mission when you meet up with a couple of marines theres a smart gun right next to them. ( the marine will tell you to pick it up if you dont at first and its the first smart gun you get in the campaign ) Theres a small barricade of stones a little bit to the right of where the marines are. Stand directly behind it with the smart gun and wait a little bit. Aliens will start coming over the wall in front of you. Now wait till the aliens start to get close and then hold LT and start firing away ( do not let go of the RIGHT TRIGGER or you will not get the achievement ) aim for the aliens when i did it they started going around the pole in front of me and coming straight at me making it easy as you only have to kill 5 of them and atleast 10 come at you straight away allowing you to mow them down with 150 bullets. For added bonus listen to Bodies by Drowning Pool gets you in the alien killing mood =].

The easiest place to get this is on the last marine level when you are on the bridge with all the aliens. Simply aim down the bridge without letting go of the trigger and you will most likely kill 5 aliens with 150 clips unless you shoot them from too far away and you miss.

Very good guide on youtube! I hope this helps you!