A little help breaking up the Holy Roman Empire.

I started my first campaign as Dublin and became king of Ireland pretty quickly. I've started working on Wales since I want my endgame goal to become Emperor of Brittania. My only problem is that the HRE has done really well, and taken all of England and much of Eastern Europe. England also became a theocracy somewhere along the line (not sure how that even happened, pretty cool though) Is there any way to break apart the Empire? I've tried plotting to kill him, but none of his vassals want to help me out. Any other tips for a noob would be awesome!