Alien Breed 3: Descent Trophies
Into the Kill Zone
Kill 20 aliens with one sentry gun.
Find a power node in a level, scroll to a Sentry Gun in your inventory and go near the node and hold down [L2 Button] to plant it. Power nodes are usually in rooms where a lot aliens will be coming at you in waves. You shouldnt have trouble getting yourself this trophy, especially since the Gun is very powerful. Kill 20 with a Sentry Gun and the trophy will pop.

A very early and easy place to do this is during the first Level Subversion. Youll be introduced to your first power node, in which the in-game tutorial will tell you to plant the Sentry Gun you found earlier in place. This room has around 30 enemies or so, so simply plant one, sit back, and let the Gun do all the work till the trophy unlocks. Note that if you don't plant the Gun, the enemies won't spawn.

Another place you can easily unlock this will be during Level 2 - Re-Entry, during the 2 minute battle where you'll be facing waves and waves of aliens.