They could just have one competent mercenary put a crossbow bolt through her heart or brain. It doesn't really need to be any more elaborate than that. Considering she's all over the place, not sticking to the royal parts of the royal castle like a typical queen, it would only be a matter of time to get a good chance. Furthermore, there would be no direct link to any opposer, as long as the mercenary hit the road and disappeared, unlike attacking the princess in a place that's under the supervision of a single party.
Still, I'm happy the opponents don't try to do anything like that. The story is better this way, not so dark.