It might have to do with the bone shard item getting multiplied by damage bonuses, but the build I'm referring to is cinderblock fish and snails (potential 2025% damage bonus).
(I haven't been able to even get close to an all level 3 run, though, so I think it might be balanced by the rng requirements)
Edit: just tested it; the bones are definitely benefiting from the snails' damage bonuses (when I killed the tentacles, the dps shot up to 85k)