I just got this achievement in Tycho's Ribs.
As soon as you enter the first area with enemies, a Lost Legion Eternal will spawn from a building on the right. I shot him a few times so he'd ascend. If he didn't become a Magus, I'd save and quit, then repeat until he did.
As an Eternal Magus, he will lob projectiles at you. You need to weaken other enemies and stay close to them so the Magus' projectiles will hit them. It took me several tries to get the Magus to kill three enemies, at which point the achievement popped.
Note: he can kill the enemies one at a time. All three do not have to be killed simultaneously.