To unlock the Careful, He Bites trophy in Borderlands: Game of the Year Edition, you will obviously need to be playing as the Soldier.
Once you reach level 5, you will unlock the action skill. Find some low-level enemies outside of Fyrestone (Skags are perfect), take down the health of a few of them by shooting until they only have a small amount of HP, and then press
to trigger the skill.
Rinse and repeat until you've killed 15 enemies and then the trophy will unlock.