I do see the problem. Through four editions, PoF was a Touch Range spell and the jet of flame shot forth from your hand and required an aiming roll. It all made sense. In fifth edition, the core concepts and mechanics of magic were refined and made more logical, but in adapting a massive body of existing spells to work with the new systems, there were several breaks in proceedural logic. ArM4 and earlier did have goofy ranges like Near and Far, because magic was not depended on a Voice to carry it.
I like ArM5, I really think it is a smoother more refined version of Ars Magica. But it does suffer from a "two steps forward, one step back" syndrome. The majority of changes are for the better, but a few things were better off the old way.
But there is hope! One day, maybe, ArM6 will refine and revise ArM5, then later ArM7 will perfect things further :mrgreen:
Yup.
IMO, in the situation discussed (and barring my sight-related* error 
), if challenge there was, it wouldn't be in "can you roust them", but in "can you roust them in time?"
Sometimes, you won't care (probably because you've acted enough in advance), but sometimes, you'll want to stop them from, say, killing the opposing army's general, or breaching into a castle. So, once you start raining fire on them, will they flee at once? Will they press on? And if so, how long?
In that situation, I'd look at the army's stats, the magus's Psyche, his good/bad stuff, and... wrong game 
This can happen with a lot of skills and abilities, where you can do the things, but, sometimes, you'll want to do them fast enough.
- bad pun intended
In my head I was picturing something like playing a Real Time Strategy game with god strikes. Herd em with Pits, kill em with flame. Take out the leadership and heroes early. Repeat until you get bored or the rousted army is to diffuse and far away to be effectively targeted.
See I almost never bid in the the auction. I'll wait and maybe become a secret second in a stat the went cheep. Then dump an obscene amount of points in good stuff. Thats me the lucky cousin everybody likes and no one can get a real read on. 
Yup, given the right spell, it'd sure look like this.
I'd bid at least 1 point in every attribute, in order to be at least ranked. Then, bid in one go what I had wanted to bid.
I'd have loved to have a player with insane good stuff. I always pictured high levels of this as a constant "atler probabilities" pattern effect going your way (or against you). You were attuned to the universe, which worked your way... But also made you a prize choice for teh great Powers.
I had a player who went the other way while becoming a champion of the logrus (which made him a kind of slave with godlike abilities). He was so much a blight on reality that he litterally dropped out of the universe, ending up in undershadow, where he wouldn't damage reality, but where all his insane powers were useless.
Destroying an army can actually be done much simpler than direct confrontation.
Enter their camp invisible and otherwise veiled, and cause a bunch of devastating diseases. One disease might well be enough to cripple an army, several breaking out just puts a complete stop to their advance. Additional benefit: It's not as obvious, thus not necessarily causing legal action in its wake.
The point is that how you destroy the army highlights your magus's character.
Yes, much better idea overall.
Of course, it takes more time, which is something you don't always have.