The boss battle at the end of Issue 1 is probably the easiest place to unlock this.
Destroy one or two electrical tesla coils in the room (by using the Glyph Bombs) so your heroes can move around without being electrocuted (don’t destroy them all, though). Enemies in groups of three will start to come out. Wait until there are two groups together (or near each other) and use Scarlet Witch’s Ink Attack (which is done by pressing the Stylus down onto your hero and drawing a line through the enemies) to defeat all six enemies. If you get all six, the achievement will unlock. It might take a couple of tries, but as long as you don’t destroy the rest of the tesla coils, they will keep coming out.
The reason for using Scarlet Witch here is so you can unlock “Six Hexxed” at the same time.