Revenge killer or scout. Easy.
Picking the moveslots is actually important on ImposterBliss though. In case you can't transform into the opponent it's either Belly Drum Illusion Slaking (resulting in a dead Blissey) or you can take a hit moderately well. I typically run Aromatherapy / Wish / Volt Switch / weather move. Since Lum Berry only works once, status-based teams can be pretty damaging, so Aromatherapy is a great lifesaver. Wish is always useful. Volt Switch lets you get the hell out in case you predict wrong or it's actually a Sub+Shell Smash Groudon and not Prankster like you thought, and the weather move is great for messing with opposing weather teams or killing/saving SturdNinja.