Solas, if any of those Fen'Harel legends are even halfway true, likely avoids conflicts of attrition, operating in the shadows as much as possible, attempting to use information to win wars - awareness of enemy supply lines and crucial defenses, knowledge of field (i.e. you do
not fight Solas in the crossroads, that's his home turf), and the element of surprise. I bet he's well acquainted with guerilla tactics. So I reckon the best way of defeating him is drawing his forces out into a conflict with another power and exhausting them.
The most obvious third power here that they might fight is the qunari. I think I'd try somehow to convince them that Fen'Harel, not Tevinter, is the real threat at play. Even if Solas is clever about what he does, and manages to keep things together when the qunari decide he's the primary target, I think that allows us to sneak into the picture whilst he's convinced the qunari are the more immediate threat, even if not the more important one.
I think, whilst the idea of getting a very, very large hammer and walloping his ass with it is fun, it's not the best way to defeat the Dread Wolf. I think you need to outplay him. Downsizing, I think, works for this approach the best. I think infiltration will also be key too - if you want to outsmart him, you need to know what he's doing.
I also agree with a few others earlier - whilst I have a love-hate relationship with Tevinter, and would happily smash its government to pieces on a good day, I think Tevinter collapsing could potentially be the worst case scenario. What if elves turn to Fen'Harel, having lost faith in the Magisterium's ability to keep things (including themselves) under control? When the state is in crisis, the populace turns to the political fringes - the qun, and Fen'Harel, in this case.