Having trouble with Trojans, T3 Thwackers, Mortafires, the dreaded Snarbolax, Mecha knights, Royal Jelly's spin attack, Grimalkins, Lockdown's Guardian shield, the Roarmulus Twins, and/or Deadnaughts?
Whip out your once underused Iron Slug and laugh as you use your charge attack to punch right through thier puny "invincibility!"
This isn't a suggestion. The hell are you trying to get at here? Make the iron slug brokenly op? make it able to kill any boss at any time?