Torgrim, at level 20, reflects every other spell, provided he is an expert in magic resistance (which is inevitable), aimed at any creature in his army. However, if you cast on dwarves, it's useless, as the success rate of the spell is around 10%. Try casting a mass slow spell on Torgrim at level 20, and you'll see that the spell won't affect all units.
I read the hero physics section carefully. It's a detailed work, but it only describes exact facts about who hits whom, and there is no information about probabilities.