Ziggurat 2

Sacrifice is the special ability of Jacob, unlocked in the 2nd chapter of the campaign.

Sacrifice gives up 5% of your health in exchange for higher damage and fire rate, with no mana spent for the duration. The length is rather generous as well so a fast firing weapon will make this easy. The hardest part is finding 15 enemies to cluster, though as with many of these ability related achievements carrots make this trivial.
I managed this in a regular dungeon room with the only enemies being carrots, though a sure fire way would be to get Maui, a guardian in the later chapters, as his second phase of fighting spawns around 20 carrots. You could similarly go into Gauntlet, weaken many of the enemies that show up, then pop his ability to mop them up.

Happy hunting (>*_*)>