Metal: Hellsinger

All kills in all modes on all difficulties count towards this total. This will come naturally through playing through the campaigns, it unlocked for me during the 3rd Hell (Yhelm) so under a couple of hours into the game.

If you, like me, played the game through when this achievement was buggy and didn't get it you can play the second level Voke on lamb difficulty. The level has about 130-150 enemies and a very easy boss fight at the end.

Just keep using Hounds and pop ultimate when ever possible. When fighting the boss save the ultimate for when he returns in 2nd phase.

To make it even easier use "Always on Beat" under Rhythm Assist Mode" in Options > Accessibility.