This achievement can be done either local co-op or online cio-op.
The big issue with this achievement is that not every boss level enemy counts for it. There are a lot of mini bosses I guess scattered through out the game.
I would have thought the Shaman of the first people would have counted but he didn't.
So far the only three confirmed Bosses that count for this achievement from my experience are Rajali the Archon and the Dapper Old Gent and the Final Boss
I do not know if the war beast at the end of Glitterdelve counts. I have a feeling it doesn't