Oh screw this. I'm not arguing against someone memeing at me.
If people want a better action economy then maybe they should use the actions our subordinates give us.
This isn't (simply) meming. His speciality, as given on his character sheet, is punching people. That's what fisticuffs is. Now, maybe BoneyM is joking and his speciality is actually the Engineering sub-discipline of Learning. However, the description of his Learning score:
Learning: 14 - Johann leans heavily on magic to skip right to the discoveries.
Also suggests that he isn't an expert Engineer. It seems like he was assigned the job of recovering warptech because he was sufficiently stealthy and survivable to recover it, and because he could cheat with Breach the Unknown and Tale of Metal, not because he's a particularly talented engineer or magical researcher.
He got that speciality because we voted to train him in fisticuffs!
Do you recall what it was before? Was it engineering then?
Unless the rock isn't magical, or the smart one suddenly comes around a corner, or the ally hands a torch to the troll beforehand, or...
It's not so much the specific hypothetical scenario that is the issue, it's the general fact that you seem to have a case of Protagonist Syndrome where you talk like Mathilde is the only one with agency, able to casually and reliably counter any tricks anyone else might have, without knowing what those tricks might be beforehand.
I grant that if Mathilde can cast Substance of Shadow a great many times without slipping up (and Boney has warned us that even mass-casting Aethyric Armor can result in miscasts if cast too repetitively to take advantage of the tirelessness),
and if the trolls are all as one too stupid to figure out a deliberate counter (which is unlikely seeing as they're sitting on Mutation Mountain),
and if nobody else, or random chance, has supplied the trolls with an accidental counter (say, carrying a half-eaten chunk of bioluminescent creature),
and so on,
then Mathilde might be able to solo Troll Mountain.
But trying to solo Troll Mountain is a really dumb and unnecessary idea in the first place!
Why would she need to case Substance of Shadow a great many times? It last forever if the effect isn't broken.
Trolls are by default either barely sapient or non-sapient. We've seen no evidence that these trolls have been mutated to be different. Mathilde is also not an idiot. She is a highly experienced and talented infiltrator. She can use Substance of Shadow to waltz into skaven warrens and steal from and assassinate their leader. Skaven are vastly more competent and security conscious than trolls, and are vastly more likely to have deliberate light sources along with funny phosphorescent rocks or creatures. Even the sapient ones are essentially living in a state of nature with no society, no teachers, no culturally embodied knowledge of tricks or strategies, while Mathilde has been trained by amongst the best of humanity. She's not infallible, but trolls are amongst the least likely to be capable of responding. Particularly as you're assuming they have some way of even perceiving what she's doing to work to counter it, and that something like a piece of bioluminescent creature actually is a fatal problem for the plan. It's just something that raises an encounter from near trivial to trivial, given she can also make herself invisible before attacking in case it seems like it would be a problem.
If you're somewhere pitch black, you'll a light coming before a troll turns around the corner carrying it. That's the same with other light sources, unless someone's given the trolls light switches. Remember that ambient light doesn't break Substance of Shadow, only direct line of sight illumination from a light source.
Mathilde isn't the only one with agency. Other people who are more reliably people can act, which is why just waiting and hoping that no one else does anything with Karag Wyn doesn't strike me as a sensible idea.
This isn't protagonist syndrome. This is a problem that falls right into the middle of the sweet spot of Mathilde's abilities. There are lots of things she can't do.