This is in Act 1 Chapter 8 'China Town'. The only way you'll be able to kill the berserker is by using the Hammer of Dawn, so this will involve luring it outside, so just shoot your lancer/gnasher/whatever your carrying, and it will follow the sound.
Once you have drawn it outside, just pick up one of the HoD targeting lasers, aim and wait a few seconds for the Hammer to triangulate. Make sure you keep your distance and target the hammer whilst the Berserker is stationary.
Once it has taken 3 or 4 hits from the Hammer, It will keel over and die, cue achievement poping.