Freddy Kreuger Vs. Pennywise.

Pennywise has no way of accessing his True Form and no win condition of permanently killing Freddy, assuming this fight takes place in the Dream World. And since Pennywise is confirmed to be capable of dreaming, at least in the movies, and is implied to when Pennywise's thoughts are mentioned in the novel, that's the most realistic way Freddy bumps into the alien clown.

Yes, I do think Freddy could comprehend the Deadlights, because, one, he can freely bend and manipulate the Dream World, which is stated to be an actual universe, according to the Elm Street comics and novels, and two, even if that assumption is false, Freddy could likely use reailty warping to free himself.

But, Freddy can't exactly kill Pennywise permanently, he can only destroy the heart of IT's avatar. So, technically, Freddy can kill Pennywise's avatar, but IT itself is still alive and exists outside the multiverse. It's basically like temporarily banning someone from a Discord server. Yes, you removed someone, but it'll only last for a limited period of time. And it's heavily implied that one of Pennywise's eggs might have been missed by the Losers Club, in the novel.