If they absolutely have to do a summoning system where the summons stay out to fight for you, I think the best way to do it would be to use the FFXI system. You summon your guy out and your MP is slowly drained while you keep him out. There are already enough similarities between the FFXI and FFXII battle systems that I think it'd work nicely.

And by the way, Ramuh > Quetzecotl.