I didn't delve into geomancy and hydromancy, as I prefered aerothurge (teleportation for usage in and out of combat, teleporting foes away from you, next to another or next to the melee; uncanny evasion as a sorts-of one-turn living on the edge) and pyromancy (haste/clear mind). But fortify and armor of frost should prove quite useful as they can remove some status effects from your team member which would otherwise be annoying or nasty. Just keep in mind that fortify prohibits teleporting effects.
At some point my summoning skill was ~28 due to some nice equipment. Of course the Incarnation would hit really hard and now and then onehit weaker foes, but I felt like spending the points in other skill lines would've been more useful and fun. If you plan to wield a bow on your summoner, investing more than 5 (=10) points in summoning will always make you wonder whether those points wouldn't be more effective or fun if they gave you access to more utility skills or warfare/huntsman damage multipliers.
I'm not a min-maxer and can only recall the experience from a single playthrough though, so my advice is bound to be incomplete – dangerously so!
