-The 3 fragments that are up in the room with Menance, count towards the 10
-I just pulled Menance, kept him tanked, and let him spawn 7 more fragments. When the last spawned, the achievment came. Took almost 10 minutes though.
It is safe to evac/succor once you get the achiev, should 10 fragments + menance be too much at once. Upon returning to his corridor, it was just him and 3 fragments up again.