[Finalgenesis]

Magic damage (just think non-physical damage, not 100% accurate but close enough) pops mistform, and thats the angle you want to go for.

As mentioned, AoE damage spells, the bigger the better, battlefield-wide even (soul drain, bloodletting, bone grind ...etc). Note that immunity prevents mistform popping against the corresponding damage type (cold immune unit with mistform will not be popped by cold damage).

Magic weapons, fire arrows as mentioned (though if enemy can cast mistform, you can probably expect storm / mist / arrow fend from any decent players, unless you're sneaky, though at most you can pull it off 1-2 times), bow of war, troops with native magic weapon such as Caelum, Atlantis, summons like unfrozen warrior (flint sword - magic), banes, wrights, fire snakes, lamias, ghosts. Thugs/SCs with AoE magic weapon like brands/scourge (note shadow brand is more then AoE 1 though MR check) ...etc.

Magic aura/shield, note heat and cold aura does not deal damage and therefore won't pop mistform. You want things like the retaliatory fire shield, banefire shield, or the more offensive soul vortex. So F or D path thug/SC (say Wraithlord), charcoal shield...etc

Preemptive gem burning, if enemy caster only carries enough gems to cast mistform once (and AI always overcast thus burn a lot of extra gems...), you can burn those up by doing a magic ritual phrase attack large enough to trigger the spell and engage in battle with them immediately after in the same turn. Say, 2-3 ghost riders.

Keep in mind that even if you bring enough magic damage to pop mistform, it will still nullify 1 magic attack, and that's a pretty big advantage that adds up. You'll likely want some battlefield buff of your own to match, Army of, WotF, quickenings, immunity + BF spell combo, mass regen...etc

Just some ideas...