The Bull

You get this achievement for killing Lawrence Barrett. It's basically a free achievement, as you have to kill him as part of the main story quest.

On the hardest difficulty, this strategy basically worked for me:

-The moment his chaingun starts spinning up, I just sprinted over and crouched behind the crates on my left. I then crouch-ran around the crates to the nearest yellow pillar.

-Up to this point I had been saving the frag mines I picked up in O'Malley's apartment during the Cloak & Dagger's side quest. I had also made a few mines by combining the frag grenades and mine templates. I then crouched behind the pillar and tossed the frag mine onto the next pillar, and hid behind my current pillar to avoid accidental detonation.

-By the time you are doing this/have done this, Barrett should be advancing on you, hopefully between the pillar you are hiding behind and the pillar with the mine. This will trigger the mine and stun him neatly, allowing you to throw another mine on the same spot. If he throws any grenades, don't worry, your mine should detonate almost instantly and blow the grenade somewhere else.

-After I used up all six mines (Barrett was still stun-locked behind the pillar at this point), I bounced an EMP grenade off the wall, and when it detonated and disabled all his augments, I came out of cover and fired one assault rifle clip (30 bullets) into Barrett.

-He died.