Solas has several things going for him: knowledge, power and a head start - he knows how to destroy the Veil, he has or soon will have the means to do it, and he's already had years to put his plan into motion. So, basically, anything that is attempted, has to be done quickly. You need to find out as much as you can about him and his plans, make allies, have flexible communication capabilities, and to be able to travel light.
1. Research.
- Find a way to get rid of the Veil without a cataclysm. It is unnatural in its origin, and one way or another someone is going to eventually destroy it, or it may destabilize on its own; better to remove it in a way that it can't be weaponized against the most vulnerable Theadans, or make it dissipate over time.
- Inside intelligence. There have to be elves who have non-elven friends and family they want to protect. Get one or two inside his operation, and try to discover how he intends to use the mortal elves (like say in a giant blood magic ritual, or something), whether or not they believe their dormant magic and immortality will be restored (is he deceiving them about it?), where they are gathering, and when exactly Solas plans to destroy the Veil.
- Dig for some truth that will put Solas off-balance, or make him reconsider (like with Calpernia and Corypheus) - like say if there was a very good reason Mythal got killed and it wasn't just a coup, or that destroying the Veil will have effects that he isn't accounting for, which will damage the remnants of his people.
- Figure out what was going on in Elvhenan when the Veil was made. Solas didn't seem particularly convincing when he said he could handle the false gods once they were free. If worse comes to worst and the Veil is destroyed, they will be free. Whatever destructive potential the Evanuris had when they were imprisoned, which Solas said they were going to use to destroy the world, it is doubtful it has been dealt with and needs to be countered with something less passive-aggressive than the Veil.
- Figure out how to protect people from the destruction of the Veil. If the prophecy in the Canticle of Exaltations and the prophecy Sandal gave are both correct, the bloody thing is coming down. Might as well be prepared.
2. Allies.
- Find other ancient elves who may be in opposition to him. The Forgotten Ones (three whom have names, Geldauran, Anaris and Daern'thal) are still out there, imprisoned by him, and are not behind the Veil, but in the Abyss/Void, which is suggested to be underground (they could be the Old Gods or aligned with the Forbidden Ones, but maybe not?). Somehow, I don't think they're going to be very friendly towards Solas, and will know how to counter his magic. Albeit, they may not be friendly towards you, either, but if they are weak like Solas was at first, they may see advantages to helping.
- Find a dreamer. Feynriel, if he isn't Tranquil, is one of the few people known who can walk the Fade like Solas, can kill or deliver messages via dreams. If he can be located, or another like him, it would be immensely useful.
- Find Valta and her Titan and propose an alliance. Considering the ancient elves hunted Titans to mine for lyrium, I doubt the Titans are all that interested in having the elves rule over the world again. Valta seems to be able to connect with the Titan and know its will, if there is a way that they can help she will know.
- Use the Seekers. Cassandra is reforming them and rebuilding them; they are more powerful than Templars and can nullify magic or set a mage's blood on fire. Very useful against a highly magic-dependent race like the Elvhen.
3. Communication.
- Set up your own network of Eluvians. The Inquisition has the plans for construction of them from the Qunari base, they need three or four in strategic locations, or built small enough to be able to move them quickly and without disruption.
- Communication crystals. Dorian gave what is basically a cell phone crystal to the Inquisitor, so they exist. Instantaneous communication is invaluable against someone like Solas, who can send messages through dreams - as long as he does not have a way to intercept the messages.