For this trophy you need to max out any of the Assault class abilities to max level.
You will get this trophy on your way to Jack of All Trades if you are aiming for that.
You only need to max out one of these abilities for this trophy.
Nano Shield- 300 Kills through Nano Shields
Drone Buddy- Kill 300 enemies with a Drone Buddy
Stun Blast- Stun 500 enemies with the stun blast skill
Whichever one you max out first this trophy will be awarded.