At the end...
(spoiler alert if you haven't seen the movie yet)
I haven't read the book(s?) or re-watched the previous adaptation of this, but is there a reason Pennywise/It is so weak at the end? I mean, he is confronted by a couple of children, why couldn't he have just ended them by turning into a huge hammer and squashing them like little bugs? I got the impression throughout the movie that he or it was a very powerful being, or was that all just illusion? It just seems like he was built up in the movie and during the third act he kinda just went limp and retreated.