I think the only way for mages to win is to fight with guerrilla-style tactics.
I mean, mages have range. Fireballs, Tempests, Storms of the Century, and so on can be used for mid-to-long range devastation... but those also tend to reveal the location of the offending mage quick smart. And the mages do not have a numerical advantage over the templars, despite Varric's insistence of "and then, FIFTY BLOOD MAGES burst out of Hightown!"
No, they need to be sneakier about it. And that means, using magic and those aforementioned sellswords for the greatest asset in armed conflict - covert ops and enemy intelligence.
Runes can be laid down and primed as traps for unsuspecting Templars. Since ancient elven Arcane Warrior techniques have apparently become so widespread as to be co-opted and rebranded by the Chantry, there's no reason the rebel mages can't have a few Chasind shapeshifters in the ranks. Don't use these in suicidal head-on attacks - work with your mundane support troops to infiltrate Templar camps and strongholds. A pair of strong flying birds, such as owls, can easily transport a set of grappling hooks, for example. Grease + any flame spell make punitive acts of arson accessible to even an apprentice-level mage. Earthquake spells can level granaries and destroy supply depots - and who says the mage has to wear robes and an enormous staff, basically flaunting their magi-ness to the world? The object, I surmised, was to win, not make some sort of doomed statement?
Send more infiltrators, who are not WEARING ROBES, into Chantry hotspots like Val Royeaux and Halamshiral. Stir up discontent among the native elven population - Val Royeaux's alienage is apparently some sort of monstrosity packed to the brim with starving, abused inhabitants, largest in Thedas, and I can't imagine Halamshiral's elves being anything but quietly restive at living in a city built mockingly over the ruins of the Dales. Leave a few nasty surprises here and there. The Chantry's leadership has just been decapitated at Haven - now is the time to strike at the core.
If you must summon demons - and I'd advise against that, since they're more likely to eat YOU as a matter of convenience rather than do what you ask them to - set them in a primed circle and leave them ready-to-summon in false camps for the enemy to find - nothing ruins a Templar's day like clearing out an abandoned mage camp and triggering a wave of demons. Cost effective, and we know it's doable since Danarius did it in DA2 in his Hightown mansion, after all.
Finally, don't stay in one obvious location for them to bring out the siege engines/complain to the local rulers. And for the Maker's sake, what has Tevinter ever done for you?!