At the start of the Character battle step, if you have no other units in the Character arena, retreat Amedda.
Put a damage counter on one of your units Gain 1 build point. Play only during your build step.
Mas's ability works only once per turn.
If Mas Amedda is the only unit you have in the Character arena and Trash Compactor or Mustafar Battle Grounds is in play, both effects trigger at the same time. If the Dark Side player resolves Mas Amedda's ability first, then Mas Amedda will retreat before placement of damage counters. If Mas Amedda is not in the arena when Trash Compactor or Mustafar Battle Grounds resolves, Mas Amedda will not get a damage counter.