Kill 30 aliens with headshots

The best weapon for this trophy is the Railgun, or the Shotgun when in close combat. Just aim for the head on every possible alien and you will have it very early in the game. The easiest enemies to get headshots on is the Pigcops, since they can't teleport and fly around like the Assault enemies can. If you have completed the games on Easy or higher, remember to turn the Big Heads cheat up to 150% for some easier shots.

A really easy place to rack up headshots is in "The Shrunk Machine". Right after you get the Freeze Gun in the beginning of the level you will enter a big room with a lot of Pig Cops coming at you. Freeze every possible Pig and perform a headshot with your secondary weapon. Don't worry about ammo for the Freeze Gun, it will automatically refill itself while it's not in use. There will be around 10 Pigs that you can freeze, so after every Pig is dead, reload the checkpoint until you have the trophy/achievement if you want to make it easy for you.