If monsters = more than 3.. I consider using magic if they take decent damage from it.... ie.. pulling alot of the snakes in the uplands together and oil+firaga...

I use Bravery, faith, haste and other green magic all the time.. I use slow alot.
As far as blizzaga/firaga/aeroga/darkga/graviga/thundara.... only if multiple enemies... and you get the spells too late to really do much before then.