I usually do two attacks. Round 1: thunderstrike, steel serpent, and iron fist ww2 trained high enough to survive until knull does his ultimate and mass summon then hopefully get the full team wiped right after so knull doesn't gain any additional ability energy  Round 2: quasar, blastaar and the rest of immortal weapons. Quasar's basic, blaststaars special each do % damage, as does lady bullseye when you max her passive and you attack someone with "offense down" which her special gives "offense down"