The monsters can, in fact, feel pain. This one, in this moment, is having an exceptionally bad time. All it's aware of is a sudden pushing force, a crushing pain throughout its body, a piercing pain, and then darkness.

The monster's body goes to pudding against the wall it's pinned into, the last remnants of it dripping off and towards Gyre. He's close enough to get a full nose-full of sulphur, rotten fish, and strawberries. Worse, because the monster's body is still touching his sword he won't be able to get it back any other way than to pull it free with his own hand.

Talk about a bad day for the monster but a good day for everyone else? Or perhaps it's just as bad of a day because of the smell. No one wants those smells, plus petrichor, wafting in the heated air. Despite the monster's defeat, the air's still pretty hot. It won't cool for quite a while.