First, ESUNA. Get Esuna on at least 3 people. If 1 gets Sporefalled, then switch in another to Esuna them.

Second, no quickenings. Even a 10-hit Mist combo usually does only about 1/3 of his HP, and it uses all of your MP.

First, go for the treants. They are irritating. Then, just whittle him down. When he uses Sporefall, just heal everyone up with Esuna. Keep healing and hitting and you will do fine.