King of Tokyo: Acid Attack
The damage will hit any valid target of a normal dice-based attack, so if you are in Tokyo (City or Bay) all players outside Tokyo are hit, but if you are outside Tokyo the player(s) in Tokyo are damaged. (Taken from the official-ish FAQ from BoardGameGeek)

The damage from Acid Attack does not count as an attack, and a monster in Tokyo cannot yield their place as a result of it. It is explicitly stated on page 4 of the rulebook:


  [Life] lost dealt from a Power card effect is different than [dice Claw symbols]. A Monster can only Yield Tokyo when losing from [Life] from [dice Claw symbols].