Serris

Serris isn't that hard to beat...kinda. First you hit it in the head(which is pretty hard). If you hit it somewhere else, like on the tail, it won't hurt it. After you hit it, it will go crazy (it glows and goes really fast).

Since it goes so fast, you think it'll be hard to dodge it, right? Well, actually, all you have to do go onto the middle platform. When it makes a large arch over the platforms, that's your cue to go hang on to the climbing thing above you, because a little later, he'll charge through the platforms.

After you hit him a couple of times, he'll turn into a Core-X. The only different thing is that every time you hit it, it becomes invulnerable and starts blinking. When it stops blinking, it will become vulnerable again.