Leveling a pure caster AT is going to be an awful experience...
You dont get your build-defining skill till L8, so you're probably stuck leveling 1-7 as a gimpy TA with no THF feats (wont have the STR). The tree itself doesnt really provide any useful attack, it just provides support, so you're really dependent on other trees to actually shape your gameplay.
And you wont get your first native AOE attack (Chain Missile) until L11, meaning you're stuck with the Core 3 SLA at 10s CD. And even then, you're having to single-target MM-immunes anyway
A single-target caster is just awful
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I really think the 100% scaling SP in the capstone is too limiting. The class itself is not complete enough, it kinda assumes you're bringing in something from multiclassing, but the 100% scaling is too valuable in Epic to keep your added damage relevant, and that requires 20 AT. 50% scaling on Ambush is not worth giving up 8 levels in another caster class (and thus their top-tier spells), so its kind of a catch-22. If you're going to gimp yourself to mid- and low-tier spells and SLAs, then you need that 100% scaling to make up for the lost damage.
I think swap it into the T5, similar to how DA gets 200% Harm scaling in T5. That's still requiring a major tradeoff, but gives you much more flexibility to multiclass to help you actually do something in the game, and make use of the marvelous Mixed Magics mechanic