Board Thread:Help Desk/@comment-5815341-20121205003338/@comment-5727701-20121220014300

I don't know how your Hellstone arena is set up, so I can't offer specific help, but I can still give some advice.

If Skeletron Prime, any of his hands or any of his projectiles are inside your character when the damage immunity from the Hellstone wears off, he will damage you instead of the Hellstone. Building a Hellstone bridge or several platforms that you can grapple to is usually the best way.