Alright, so about Penny's special and unique defense.
We know already that Pennywise's shape is something it has to comply to. Depending on its shape, it has a different "potential" weakness that collective, unified belief can make real. The most obvious example was its werewolf form; the Losers collectively believed that silver was a viable weakness against it, so it was therefore harmed by the silver. To physically harm it (or prevent it from regenerating, anyway), you need to have multiple people believing in the one solitary thing being used against it.
But there's a problem.
When faced with multiple people, it is capable of assuming multiple forms at once, effectively making it so that everyone perceives it as something completely different:
Richie chanced a glance behind him as he flung himself onto the package carrier and saw the Werewolf crossing the lawn toward them, less than twenty feet away now. Blood and slobber mixed on its high-school jacket. White bone gleamed through its pelt about the right temple. There were white smudges of sneezing powder on the sides of its nose. And Richie saw two other things which seemed to complete the horror. There was no zipper on the thing's jacket; instead there were big fluffy orange buttons, like pompoms.
So once again Bill Denbrough found himself racing to beat the devil, only now the devil was a hideously grinning clown whose face sweated white greasepaint, whose mouth curved up in a leering red vampire smile, whose eyes were bright silver coins. A clown who was, for some lunatic reason, wearing a Derry High School jacket over its silvery suit with the orange ruff and the orange pompom buttons.
Bill remembered himself and Richie in the middle of Neibolt Street, after the thing which might have been a clown or which might have been a werewolf had disappeared, embracing each other and weeping.
So what does this mean? Well, it means that a weapon which would normally work on one of its current forms would fail, due to the other form not having that weakness and due to it having assumed both forms simultaneously. The only way the Losers were able to get around this was by being psychically linked to each other, which allowed them to collectively perceive its form as being one thing, trapping it in that form.
So, in essence, physical attacks (even ones that would say, target its insides or ignore durability in a way that doesn't involve a concept or a potent form of hax) are out of the window with this thing. Every character here will perceive it as being something different, with small shades of its other forms leaking into each person's perception.
My space-clown friend has some offensive hax as well that I managed to find quotes for in a blog. Who wants to see?
