An idea I once had for spirits was to just entirely remove the Spirit's AI and make them untargetable. In exchange, the summoner gets access to a select spell library dependent on the spirit currently out and about. Fire Spirit active? You have (as an example) Fire I-III, Burn and Pyrohelix, but their damage modifiers are all derived from Summoning Magic skill with a small penalty in additional MP costs on top of spirit perpetuation. The downside is that none of that is hate-free (which is basically the benefit of having avatars out).
Spell lists and overall numbers would have to be looked at, but that'd be the gist of it.