Purify the Purifiers
Defeat 2000 Purifiers.
This is likely the easiest of the kill related trophies. Purifiers are the basic enemy type you fight from the very beginning of the game. You will likely kill quite a few just by playing the game and you will defeat even more while you farm kills for the rarer enemy types listed below.
If you are struggling to get Purifier kills, the challenge mission listed in Reinforced
allows you to get close to 100 kills each playthrough. Additionally, during the closing credits to the game, you and your faction are in an arena full of basic Purifier soliders. The credits are long enough that you can use your Ultra power multiple times and defeat a couple hundred enemies before the challenge ends (you also receive a cool Retro outfit for your character).
Defeat 20 Purifier Stalker Mechs.
Purifier Stalker Mechs are the large 2 legged walker robots that first appear midway through Mission 4. These robots have 3 basic attacks: a large stomp that produces a shockwave, a series of small stomps when you get too close, and a barrage of bullets from its machine guns. To damage these robots, you must first attack their legs until they collapse and then attacking their backside will do damage to their health.
You do not encounter many of these throughout the game, but after the game Stalker Mech kills are best farmed in either the Surge challenge mission from Mission 4 (Row 2, picture 4 on the Select Challenge screen) or the second Purifier challenge mission found in Mission 5 (Row 3, picture 3 on the Select Challenge screen).
Defeat 20 Prime Enforcers.
Prime Enforcers are the Purifier enemies that appear in suits similar to the boss Cameron Hodge. There attacks are all the same as Hodge's and like Hodge, you must first destroy their shields before you can damage their health.
You first encounter them in the prison break style challenge mission given by Quicksilver or Colossus in Mission 4. This challenge consists of you and your faction ally fighting 100 Purifiers on a helicopter pad, with 1 Prime Enforcer that arrives towards the end of the fight. After you play through the game, this is the best challenge mission to farm kills on. On the Select Challenge screen, it is the first mission in row 3, featuring a picture of both the X-Men
U Mad, Bro?
Defeat 30 U-Men in the Secret Lab.
U-Men first appear in Mission 4, and are the enemies in giant bubble helmets you encounter in the hidden laboratory. While you don't encounter many of them throughout the game, you can farm U-Men kills in the U-Men challenge mission you pick up in Mission 4. On the Select Challenge load screen, this is the second to last challenge in row 2, and pictures a U-Man with a red X covering him.
The Goon Squad
Defeat 500 MRD Troops.
MRD Troops are first encountered towards the end of Mission 6. They come in two varieties, soldiers with mutant powers and miniature looking versions of the Sublime boss from the U-Men lab.
The good thing about these enemies is that you encounter a TON of them in Mission 7. If you need to grind out MRD kills, the best challenge to do it in Forge's challenge mission (it is the very last challenge mission in the Select Challenge menu). In this mission, you and a reprogrammed Purifier Stalker will fight off a giant horde of MRD troops. There are plenty of blue crates around to recharge your M-meter so let loose with your Ultra Attack often to rack up a large number of kills quickly.