I've been trying to kill an heir now for about 10 turns since he's an odd-toed-gluttonous-raving loon. with 0 command and 0 acumen. His younger brother kicks butt and I want him on the throne.

My assassins all fail. I tried burning him with an inquisitor, but because I constantly crusade the heir has high piety and passes.

I tried sending his unit in alone against 2 stacks of enemy troops. Every single time he retreated safely. The enemy just could not capture him. I tried autoresolving, and he comes back. I tried playing it out, running him into the middle of a converging enemy, having his unit surrounded on all sides. It gets down to just him alone and the defeat screen pops up suddenly. He somehow escaped and made it back to my province.

How on earth can I make him dead?