The main one for Plasma would be quiet a nasty one. She would kill them from the inside out, though she would need to create a hole to put her hand through before melting, burning or freezing there insides and whatever else she can do with her change of elements.

The other is just nuke the shit out of them, slowly or quickly. 

The rest would just be a mixture of things.

But in the end if she were to fight one of the X-Men she would probably not do it, well if they were one of the older students or teachers.