Metro Redux (EP)

The Metro Wiki defines a "Mutant" as "generally tough creatures which have adapted to their nuclear environment and can withstand high levels of physical trauma."

So basically killing almost any enemy that isn't human will count towards this. If you want to grind this out quickly, load up the mission "Cursed". Near the beginning of the mission Khan will tell you to go find a bomb, and he will stand by two soldiers. Go straight down the platform from here, and on your right you will pass three steps of stairs, go up any one of them and you will come to some escalators, which are destroyed. As long as you don't complete the objective I'm pretty sure mutants will just continually spawn here, so kill away until your achievement pops.

However it is more than likely this will pop before the game ends, as you fight many mutants throughout the game.

Mutant includes all non-human enemies. You should get this in one play through, but if you don't, there are many places to grind it out. Remember that mutants have less health on survival difficulties.

Dark star is an excellent level to grind many achievements. Load it up and use the flamethrower on waves of enemies.

Cursed is slower, but the enemies come at you in small groups.

On hunter, you can grind mutant kills by reloading the one combat part of the level. Most of the enemies are behind grates, so they are easy targets.