Easier than it looks, all you have to do is kill the throne, then stand next to it as it's death explosions go off and you'll easily be able to sit on it while headless.
Alternatively, you could carry an explosive (or blood, really any weapon capable of killing the user) weapon and then after the throne dies just use it to kill yourself and sit on the throne.
(Note that the strategies involving killing yourself with explosions don't work with boiling veins, so if you're wanting to do them remember to avoid picking boiling veins.)