Hunted Demons Forge

I actually ended up doing this achievement with the AI, where I played as E'lara. If you're starting a single player campaign and want this achievement, you need to focus on two abilities - Pyre Blast [PB] for E'lara and Caddoc's Wind of Wrath [WoW].

While progressing through the story, play as E'lara and boost your PB as early as possible. Furthermore, only boost Caddoc's WoW and nothing else. When your AI partner gets swarmed with enemies (which happens fairly frequently), he'll occasionally trigger WoW and you'll see them all rise slowly into the air. When this happens, quickly switch to PB and shoot it into one of the enemies dangling helplessly in the air. If you're playing on an easier difficulty and your PB is sufficiently leveled, then you should splatter all the enemies in the air.

Keep this up and this achievement should be yours by around the beginning of Chapter 3, although YMMV.

I found quite a good way to get this was to do level 1.3 where the inn is getting attacked by a ballista.have caddocs wind of wrath and play as elara.kill all crossbow enemys and jump on the ballista when caddoc tells you too.if you just sit and wait caddoc will use his ability up to 3 times and just blast em with the ballista when they are in the air play to the save point and rinse and repeat got the achievement after 5-6 plays of this level