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I want its HA to be something camouflage or sniper related - allowing it to attack enemies past protect/substitutes or you could go with an accuracy route and give it a clone of that ability that boosts accuracy whose name I forget
Or you could do something like a surprise attack where it gains priority on the first hit it makes if the opponent's pokemon has entered on the same turn, effectively giving it priority for the turn if you lead with it or making for a very strong lategame sweeper where it would just be able to chain attacks with priority as long as it was able to finish each weakened opponent off in a single hit each.
I really like that last idea.