Red Ronin

The only place where you can get this achievement is in Chapter 7:Red Ronin, just before the boss fight. Here you'll have the arena, also connected to the "The Crazy 20!" achievement, where 20 enemies will come at you non stop, of course you can't stay completely still or take too long to kill the enemies otherwise your combo counter will go off, also a big warning: I tried the achievement twice before getting it, the first time I killed all 20 enemies with a single combo and didn't get the achievement, tried again stopping at 16x or as the game says Max Combo and the achievement popped, so be careful.

Video for reference with both tries: