Batman vs. Pennywise? Sorry, but I think the clown would find a way to get into Batman’s head and torment him without end until the Dark Knight finally cracked. There’s no real way for Batman to anticipate Pennywise no matter how much he reads up on him, or how much knowledge he gained of Derry. The maniacal clown is just too much for anything born of earthly means, and would break Batman down methodically, perhaps even going so far as to prey on those closest to him. Why not? The Joker did it for years and he held his own against Batman in more ways than one.
But wait you say, Batman isn’t a kid. Would Pennywise really be able to affect him at all?
Think about this. Bruce Wayne is still traumatized by the death of his parents after all these years. He still revisits that scene again and again in an attempt to either torture himself or discover just what he could have done differently that. Recall that in more than one version of the hero’s past that he blames himself for the death of his parents, and holds that guilt close to the vest so to speak. What in the world do you think that Pennywise, a creature from beyond our dimension, would do with that?
So Batman’s not scared of clowns, big deal. Pennywise has a lot more forms than this as we’ve seen. He can be pretty much anything he needs to be in order to get to his prey. Becoming a giant bat probably wouldn’t do it since Batman has mastered that particular fear. But preying on the Dark Knight’s most obvious mental weakness? It’d be the easiest and most obvious target, and we know by now that Pennywise isn’t the king of subtle. The only problem with this is that as I said above, Batman cannot anticipate this opponent. Pennywise can come at him from so many different angles that Batman wouldn’t know which way to turn.
In a showdown of fictional characters this one is among the great mismatches of our time. Even Superman would have a problem with Pennywise since there’s no telling just how strong the entity behind the clown is, nor if Superman would be affected by the dead lights that we saw briefly in the movie and prominently in the miniseries back in 1990. Batman wouldn’t be immune to these at all being human, but if he glimpsed them once and lived to tell about it then yes, he’d be prepared the next time, as much as he could be at least.
Batman is all about planning, knowing his enemy, and working off contingencies when the straight up fighting techniques don’t work. He knows how to come at his opponent in many upon many different ways, but aside from a few of his enemies he’s never met anyone that could so masterfully use his deepest, darkest fears against him as Pennywise could. I know that some of his enemies have messed him pretty bad inside and out, but Pennywise wouldn’t be so worried about breaking the bat. He’d be more interested in driving Batman insane with fear before the final curtain.