Would Troll Ascetic be protected against petrifiation counters from Xathrid Gorgon ? The petrification ability it's not controlled by the opponent that controls Xanthrid Gorgon but by the creature itself...
The controller of a spell is the one who cast it (or the owner of the card/copy representing that spell if there's no controller), the controller of an ability is the player who either activated it, for an activated ability, or the one who controlled the source the triggered ability triggered from.
For an activated ability, not that some may let your oponnent activate them, in that case your opponnent would be the controller of those specific abilities.
Rules question? Have you read the Basic rulebook already? No? Why not take some time to do that?