If a spell cast on you is momentary, you basically have two choice with vim:
- destroy it => perdo
- alter it => muto
Muto will require you to act when the spell is formed, before it is cast and that it's not a spontaneous spell. You would have to penetrate the spell penetration.
Perdo will need a beating initiative (but so would Muto in fact) and you beat the level with the die roll. You wouldn't have to penetrate the spell.
RegoVim doesn't work on momentary, because you canno't suppress it. Suppressing cannot be destroying because that's perdo, and there is nothing to suppress: momentary is the shortest duration.
You may be tricky, and create a false "you" (whom he would think he is targeting), acting as an intangible tunnel on him, using ReVi anchored to a Imaginem spell. That may or not work depending on SGs.
And, yes, counterspelling is something where spontaneous players are better, because it's essentially that: "pouf" (i create the perfect "anti pouf") !
"paaaf" (quick, now a perfect "anti paaaf")
If your enemy is smart, he willtry to cast multiple spells in the round (requires high score for multiple fastcasting in finesse), multicasting (not IMO since I intend multiple target for it), or even items effects triggered all together. He could obfucast his spells, shroud them, mutovim them to divide them...
Or, he will try to counterspell your counterspell.
I have a NPC whose specialty is exactly this: casting a spell very quickly (we are talking of initiative rank 18 or more), then counterspelling (he has some formulaics for it and a focus for spontaneous) your counterspell, with fastcast initiative of 30 and more (using LoM rules).
A real hermetic combat is usually... deadly.