I know this is a bit of a thread necro, but I thought I would add my two cents to this as I just completed both RoT and PS with a Geode. I know when I started I couldn't find any Geode specific info online and seeing how I don't speak German, the German wiki didn't help me out too much. Maybe someone someday will find this thread and get some info out of it
I played my Geode a lot like a D&D Druid, that is, a secondary tank with some buffing spellcasting. As a pure spellcaster, the Geode is kind of weak. No attribute buffing spells, no Balm of Healing, no summons until an optional late game quest, no Ice Cold Warrior, and very few direct damage spells.
The Geode specific spells are kind of a mixed bag:
Body of Ore: Probably my favorite Geode spell, I had this one up almost the entire time. It creates a damage of time aura around the Geode that has a chance to knock down enemies for even greater damage. If you don't plan on meleeing with your Geode though, this is useless.
Forces of Nature: Geode's only healing spell and it's really only for emergencies seeing as how it uses a huge chunk of AE and almost all of your ED. It does heal your whole party though.
Sumu's Shield: Nice buff that increases the armor of your entire group, but only for a short amount of time. I spent a few boss battles doing nothing with my Geode but casting this over and over.
Protection of Earth: Highly situational buff that protects your party from burning, poison, and being knocked down. Makes a few boss fights almost trivial, but not much use outside of those.
Strength of Earth: AoE damage spell with a chance to knock down enemies. Meh, I didn't get much use out of this. Half of the time I would target an enemy or group of enemies with this, and by the time I actually cast it, the enemies would wander out of the area, causing the spell to miss. It's not bad though if you do actually hit.
Body of Sumu: worst spell ever. It's a single target direct damage spell that causes wounds on your Geode. The only time I could see this as being useful is if you are having problems with a boss and you have another party member who can heal the Geode's wounds. That's it, really.
And that's all of them. IMHO they don't really make up for the spells missing that an elf or human get.
I specialized my Geode in staves and focused mainly on Defensive combat skills. Looking back I should have picked a one handed weapon and shields to increase my parry.
I played through most of the game with Forgrimm, Cano, and Fayris. My general strategy for trash mobs were to have my Geode, Forgrimm, and Fayris's pet tank while Fayris and Cano picked them off with bow and crossbow, respectively. Boss battles were pretty similar except the Geode would cast Sumu's Shield constantly.
That's about all I can think of right now. Honestly, the combat in this game is easy enough that most reasonable strategies should be able to succeed.
Hopefully someone gets some use out of this post. I had a great time with this game. I even somewhat enjoyed the add-on.