What about not making a summoner class at all...instead do the avatar battles and "unlock" that avatar and give an ability called "summon" to all the classes. once you activate that command you get a list of all the avatars you defeated and can summon one....setting off a 2hour or however long cool down timer.

What happens when you summon the avatar can be traditional summoning when they come out...do their thing....then leave.

Just an idea~