Martial arts practice. It's getting a little psychologically easier each time, as his mind accepts that injuries to himself and others just are not permanent.
He can't feel about pain what Carissa feels, he doesn't think. But there's a triumph in making it through, walking away healed, step by step becoming somebody who's Been In A Fight.
...the thought of fighting Meritxell at all, let alone Meritxell transformed into Carissa, still seems flinchy. He might possibly need to equip Meritxell with a dagger, or something else that makes her look more dangerous, and be attacked by her first, before he'll feel okay about fighting back.
It'll probably go away, that hesitation, after the first time. Things like that are usually like that. Right?
Using Bull's Strength and Cat's Grace to train martial arts is like soaring. Keltham would say, like flying, but eventually he'll be able to Fly - or cast it from scroll - and probably that'll be a totally different sort of soaring experience.
At 4th-circle, Keltham can't sustain those through a whole training session - nor would he, he should also train fighting unenhanced - but Keltham begins and ends with them, each time. Because elementary hedonics.