If Slendy only really has one way past Penny's regen, then it's a matter of whether or not he'll use that before Penny does...well, a lot of things, really?
Considering this is a Fact Book character Chuck Norris' departure notwithstanding for the moment, I doubt he has any "in-character" moves that he's guaranteed to start with. Because of that, Penny is more likely to put him down in some way before he can use the one thing he has that gets past its regen.
Of course, this is assuming death manipulation even works as a permanent means of killing Its avatar. If not, well, Penny just sort of bounces back from everything Slendy throws at it until it decides its had enough of the smart-suited eldritch horror's nonsense.