I'm trying to find a marill with sap sipper in ORAS, and am using horde battles with cotton spore, but am unsure as to whether or not cotton spore will proc it. Will it?
When a Pokémon with Sap Sipper is hit by a Grass-type move, its Attack is increased by one stage, and the move will have no effect on that Pokémon.
Source
Because Cotton Spore is a grass type move, Cotton Spore will activate Sap Sipper. Hope this helps.