Buy Quick Revieve and make your way to the power room. Not too long after the power is on he should come.

When you see the PT coming towards you, hold a grenade above your head. You should be revives with the mustage and sally.

Use the MandS to kill him and you can get the achievement and keep both your guns.

Tip: try to kill him close to the quick revive so that if you want to rebuy it its easier to get to.