So this one is kinda interesting. After one or two attacks and clairvoyance Pennywise will realize Freddy can't really be scared like most other humans. Once that happens either Freddy finishes him off or Pennywise mindhaxes with Deadlights.
Since Freddy is more likely to toy with him, especially since he would love the idea of defeating an Eldritch Creature such as Pennywise, he would laugh or blow off his legs before actually finishing him off.
I give it to Pennywise because his Perception manipulation is so potent that boxes Freddy in so he Deadlights him or while Freddy is laughing or making jokes (like Richie) gets caught in his Deadlights (just like Richie)