[Slydria]

Level 60+ Corrupters can get some 80+ RES making them tough to deal with. And they were already a pretty tough enemy without it, thanks to their spells which deal reasonable damage and can apply potent debuffs.

Evokers and Priests don't really have a problem against Corrupters but...

Other mages struggle. Lantern Bearers, Hexers and Summoner promotions deal pitiful damage when up against the high amount of RES. They also struggle to apply their additional debuffs. Most youkai target RES with their skills, making most of them a liability against Corrupters.

Now I'm not saying there shouldn't be monsters that give Mages trouble, but, I think Corrupters are a dangerous enough enemy and them having high RES limits the counters that can be done against them. (E.g. Silencing would be a good idea but because of the high RES, spells like Black Bubble and Drown are likely to miss their mark.)

I also don't think giving a monster oodles of RES is necessarily a good way to make an anti-magic monster as it greatly punishes some mages but is barely a problem for others.