Dragotha killed two of my players last time. They planeshifted to Magepoint and resurrected two of them.
Tonight, they went in with "Hide from undead" potions, which are 50 gp each. The wizard snuck in, cast dimensional anchor, and then opened up a gate in the ceiling above Dragotha. The gate opened up a portal at the bottom of the ocean in Acadia, which is an ocean of Holy Water (2d4 damage per quart).
On the fly, I didn't know if I should finish the battle or if they would have fun with more combat. I just subtracted half of his hit points before Dragotha seals the gate with a wall of force. The dwarven fighter teleported inside Dragotha's rib cage and attacked from there, where the breath weapons wouldn't really reach. Ultimately the dwarf severed Dragotha's spine with his axe, having two halves of the dracolich fall into the pool of holy water (they put a wall of force stopper over the chasm leading down into the wormdrake lair.