I don't know if we are supposed to be speculating or not, but since everyone else is doing it, I will too. Judging by the flavor description, I think Mega Sableye might get an ability that raises its defenses each turn (like Speed Boost). It would definitely "take advantage of any opening" since getting that first defense boost would probably make it very difficult to kill and it would eventually get to the point where "no attack can even make a scratch" (at least it would be extremely difficult to kill a Mega Sableye with +6 in both defenses). Of course, this is complete speculation and Mega Sableye's actual ability could be very different. Also, Game Freak doesn't seem to like defensive Pokemon that much, so I am probably wrong.