@Riger I'm pretty sure he's always been that strong. At the start of the manga, he felt conflicted over his status as a monster, which, in my opinion, caused him to hold back from using his strength. That psychological block was negated whenever Clair was inside of him, controlling his body. But that's no longer the case as he accepted his status now, and decides to use his abilities to stop his former friend.
@Kasanimo
Maybe...
But the sheer amount of devastation to the monster's face in this chapter makes it look like he has gotten a lot stronger. Especially considering how little damage he did to the 'runner' girl who was mostly human in her monster form. Even when being controlled by the blonde girl the hits weren't that devastating.