You can find this enemy only on chapter 3 and he is not always present due to the random nature of the game so, if you are hunting for this specific achievement can be a pain. This dude vanish in 5 turns and have 999hp.
The way i found to easily kill him is to use the character "Silent" and go for a poison build with at least two "catalyst +" and one "burst +" and than do the combo. The relic "runic pyramid" helps a lot to setup this combo quickly.