Well i went for force of nature anyways. Tree form has already got good defense, lots of hp, damage reduction, ect. Fairy fire gives avoid, probably around 20% when maxed and 1s duration trail which is meh unless you are loaded with on striking procs. FoN gives fire/cold/light dmg depending on str and since most builds dump loads of points in it = more damage. More damage = dead enemies faster = best defense.