This is a very long battle.

What I would reccomend is subbing in your inactive party members to wait around to lure the treants closer to the elder wrymn and then use a quickening chain to hopefully take them out. Then sub in your main party (you can just wait for those characters to die) and just whittle him down. Have at least one party member with a ranged weapon as your leader, to run around and regain MP and avoid sporefall. Everyone should know Esuna. Keep Larsa alive, because his hi-potions are love.

I ended up using quickenings with my main party, too, so was short on MP the whole time. It still wasn't too bad, because outside of sporefall the elder wyrm really just focuses on one character. I disabled all cure gambits in order to conserve MP, and just relied on several phoenix downs and Larsa's hi-potions to keep me healed, which ended up being enough. The trouble was running around with Ashe with a bow to be able to cast Esuna (because everyone else typically got hit, and often confused). But outside of sporefall, the elder wyrm really isn't bad at all. It just takes a while.