Bumping this because I have read this manga too and now I can't find it anymore. I spent in total a whole day trying to find it and could not and now I want answers.
The battle against the tamer happened during one of those dungeon explorations where each party member is left behind to deal with some powerful enemy while the others keep going. This battle was the second to last, or something, and it went like the nun revealing her true power after everybody left, she battled the tamed undead monsters and cut one arm off the tamer.
The tamer is a woman who disguises herself as a merchant to exploit a dumb prince. This prince did the cliche engagement cancellation with his fianceé to get together another woman. The ex-fianceé is the main character of the manga and she leaves the country after this event. She is powerful and even the king of her previous homeland can't freely go against her.
It shouldn't be an isekai, at least not for the main character. I don't really remember if any other character is involved in that trope.