Activate Goblin Bombardment by sacrificing Optimus Prime, Hero.
When Optimus Prime, Hero dies, its second ability triggers, returning it from your graveyard to the battlefield as Optimus Prime, Autobot Leader.
Activate Goblin Bombardment by sacrificing Optimus Prime, Autobot Leader.
When Optimus Prime, Autobot Leader dies, Mikaeus triggers, returning Optimus Prime from your graveyard to the battlefield as Optimus Prime, Hero with a +1/+1 counter on it.
Resolve both Goblin Bombardment abilities from steps 1 and 3, dealing 2 damage to any target.