It's just the combination of keeping the room lit, destroying the small squid that spawns, then avoid the things he spews out, finally attacking him and avoiding his tentacle while shooting.
Which is annoying.
Kind of the same with the upside down lobster thing in Ice Veil, I was trying to get his bomb near him everytime dropping it from the top while escaping myself. Well, The bomb didn't stay near or the lobster was already gone, so I just bond it to me, shot the lobster from the roof and stayed above him, being sure that I hit it.