Right now, Solas has only half or less than half of his power from the days of Elvehnan. But if worst comes to worse(which often happens in games like these) he does get rid of the viel, and becomes nigh omnipotent. Now, he can do almost anything, and has protection against beings with similar or the same powers(how else could he imprison the others?), and can do feats that are similar to the most powerful Comic Book heroes and villains(like survive the ending of reality) How can he be stopped?
I see two options for dealing with this.
1. 4 reveals the source of Solas's(and the rest of the Evanuris's) power. The next question is, how well protected is this source of power?
2. Outsmart Solas. Perhaps in a ironic plot twist, you defeat Solas not through power, but outwit him. Plus, Solas, Solas outwitted his fellow mages, Corphyeus, and even the Inquisitor, possibly even manipualting them like chess pieces. So he'll be defeated by his greatest weapon.
Of course it's plausible that this scenario may not happen, but in DA, you can't be careful. And in DA, the worst often happens.