X-Men: Destiny

U-Men are the enemies in the green like hazmat suits with the glass helmets and have mutant like powers during Mission Four "Mutant Rescue". There are the normal ones and then the bigger ones which can be distinguished by a mini-boss health meter at the bottom of your screen, either are acceptable towards this achievement.

If you want to grind this achievement; during the fourth mission you'll come across an area in which a couple of these U-Men take a female into a brightly lit doorway in the back right side of this room. Clear the enemies in the room you're already in and then instead of following the "X" marker, head into the doorway with the bright light the U-Men took the girl into earlier, this will be a challenge area. You'll encounter more U-Men in this challenge and once completed you can load up your save and select any challenge you have completed to play it again. This would be Challenge Mission #7 (images from left to right) to grind this achievement fighting U-Men.

A couple of goes at this challenge and the achievement is yours.

I chose to grind this achievement by doing Challenge Mission #8 (images from left to right) again and again. To do this, hit "Load Game" instead of "Continue" at the beginning and you will be allowed to replay a mission or a challenge. In this challenge you get approximately 30 Purifiers, plus a U-Man and finally a Purifier Stalker at the end (helping to boost all of those achievements). As an additional bonus, you also get an XGene for each challenge that you successfully complete (as well as XP).

Alternatively, you can re-play Mission 4. I only had 12 U-Men defeated after one play through so that is at least two and a half playthroughs of the mission.