I was not here for about 2 months. Is it just me, or did the challengers became a lot harder to fight after they all resetted?

They didn't just reset them. They did a major revamp of weapons, spells and modifications. Topic here: https://subeta.net/forums.php/read/932351/Battle-Revamp-Changelog

They should be about the same or easier. Make sure to read about the changes to freezing scrolls and freeze/steal protection mods here: https://subeta.net/forums.php/gotopost/66961752 If you’re not aware of those specific changes, I imagine it would make things harder.