Q: If Ultron Stage 3 is in play and there are drone minions in play, will actions that deal damage to all enemies (like Energy Daggers or Ground Stomp) do damage to Ultron?
A: No. Ultron’s invulnerability is absolute, which means any damage that would be dealt to him would be cancelled while drone minions are in play.
Note that this holds even if that action would remove all drone minions from play; since all damage is dealt simultaneously, and the drone minions are in play when the damage is dealt, the damage would not be dealt to Ultron Stage 3. -(Developer Ruling, Hall of Heroes)
Q: If I kill the last drone with Overkill, will the excess damage deal damage to Ultron Stage 3?
A: Overkill will work because it defeats the minion first and then excess damage is applied to the villain. -(Developer Ruling, Hall of Heroes)