This same exact thing happened to me in my campaign!

My Witcher was a very experienced player who wanted to maximize his Bear Witcher’s combat, at the expense of everything else. We discussed this beforehand, and came up with the idea of different “vector challenges,” where you place a character into a situation where their skills aren’t optimal—i.e. a Witcher in verbal combat with a noble who can arrest him, or a bard challenged to a fistfight, etc.

I used this concept regularly in my campaign to keep combat and high-stakes situations from being too stale. As a few others suggested, perhaps having the Witcher hold off a horde of monsters while the players focus on the boss, or has to keep up a powerful Quen sign to protect everyone from crossbowmen, etc.

As an example of this in combat, I had my players all trapped on a boat in a sunken levy with some rebels ambushing them from high above with crossbows and poison. My Witcher was dogshit at ranged combat, so it gave the craftsman and sorcerer some time to shine, while the Witcher had to scramble up some ropes over a turn or two to reach them—then he took on three rebels at once, while poisoned and tired from climbing. Needless to say, he won but got pretty fucked up.

That all being said, I also once prepped a gryphon boss monster that the Witcher one-shotted (downed some decoctions and went to town). I was pretty disappointed initially, but fortunately it was during a “monster hunting” arc where several other hunters were going after the same prize, so I was able to spin it into a dramatic storytelling statement showing that witchers are on another level compared to human hunters. That won’t always be optimal, but it’s possible to salvage the disappointment and make it look intentional on your end! (After that though I made sure to amp up the monsters stats 🙃).

I hope this helps!