Sniper Droid gets past Heir to the Jedi in the GC

If you mean that it kills off toons that have Damage Immunity, it depends. If the Droid is using it's 1st Special while at 5 or more stacks of Charge (or whatever), it instantly kills off the toon. The only way to prevent that is to have Instant Defeat Immunity. If it's killing off toons that have Damage Immunity while the Charge stacks are at less than 5, then most likely it's a bug. Also worth mentioning that enemies in GCs are often ramped up in terms of stats, especially Sniper Droids in Separatists GCs because of the modifier.