this one's pretty bugged, eh. or maybe it's just the description that's wrong. either way, it's not 750000 enemies, and the 100000 achievement didn't trigger once I reached that mark. do the math (crate certainly didn't...)
PS: there was no in-game announcement of this either when I reached 100k
EDIT: NOW I get it. THIS is Unequaled slayer (kill 2000 monsters with a star). it's a complete mixup for some reason! it's a problem with this site after all, not with the game