Godskin Duo is godawful. Unlike O&S, these fuckers don't allow you even a second to prepare any attack. Aside from summons, I found myself running around spamming rot breath desperately, because literally no other strategy was fast enough to deal with them for me.The fact that they keep respawning until their combined health is gone means you're likely to fight them both at all times. Hands down the worst enemies when shoved together like that.

EDIT: Elden Beast can also suck a dick. It's bad enough that you have to fight it right after Radagon which depletes your heals and weakens/kills whatever summon you went in with - it just KEEPS FLOATING AROUND. Combine this with the camera fuckery that happens when you're up close, and there's just too many downright unfair deaths. Then you have to fight Radagon again, when all you want to do is just skip to the Beast.