So i tried a bunch of different builds for pure caster and I figured out what you need to do to optimize it.

Some of the things are obvious like take spell damage stuff when possible but the trick I ended up with is how to generate enough mana to cast lots of spells.

Basic Concept

You need 1 rank in water magic for the spell that causes your towns to generate 1 mana per essence.

You need the racial trait that lets you build the town improvement that gives +1 essence.

This means your towns will all produce 2-3 mana fairly early on and add to that your other sources of mana like magic nodes and your base 2 and you quickly produce enough mana to cast offensive spells.

Other Character Creation Specifics

Don't take the talent that produces 1 mana per turn, it is not nearly as good as the starting item that makes your spells on the equipped champion free to upkeep. Take things like 2 HP/level, 1 spell pen/level and +10% exp, and even consider +3 weapon damage so you can cruise through the first 5 levels.

When crafting your race take all 4 disadvantages, they are irrelevant except research and you can get a better research choice from the racial advantages list.

Take the scouting racial advantage, and the magic staffs racial advantage because it gives you the +1 essnece city improvement.

Take 2 ranks in fire or air and get it up to rank 3 asap because you need to use fireball or lightning bolt to kill stuff. Lightning does more single target damage so you will need some melee units to clear out trash, fire does better aoe damage.

You can instead learn fireball from the magic tech tree but you still want 2 ranks in fire for firebolt for single target dps.

Mike.