Give them both Midnight Oil (Geo) and the Bully talent.
I actually recommend giving both of them Hydro over Geo for a Ranger.
Right at the start of the game you get Rain which you can stun people with using the first lightning bow you find. 100% hit chance from stunned enemies is better than slowed enemies. Slow Current and eventually Mass Slow can trigger Bully for you. Ice Elementals and Ice Walls are superior for creating terrain that can control the enemy and knock them down, which also triggers Bully. Later in the game you get Hail which can prep an entire battlefield with ice before you even start a fight. You have a handy constitution booster which can give you a higher maximum AP to combine with Glass Cannon. You have access to healing spells, and a very solid cure spell, and Freezing Touch is just great all around.
