Best ways to kill a Tier 1 caster without being one

[BeholderSlayer]

Conjurer 3 / Master Conjurer 2 / Incantatrix 10 / Archmage 2 / Iot7V 3

Orange Ioun Stone, Rod of Greater Extend x 2, 9th level Pearls of Power X 2, other things i haven't thought of.

Persisted Foresight via Incantatrix. Persisted Shapechange as well.

Centaur boy appears. Greater Celerity. Cast Extended Time Stop via rod. Cast Gate and call a CR42 creature which is good at grappling. I'm being generous here, i can easily get a caster level greater then 21. Rinse and repeat. Enough big bad bastards can grapple him without trouble. Shapechange in to an illithid and proceed to eat his brain.

I'm surprised you even bothered to dig that up, as it would be solidly stomped with little effort by a decent spell caster. Ironically, it's using mimicked spellcasting to attempt to defeat a spellcaster, and even then loses before the fight starts.

I think that this build might be sufficiently optimized to bump it up a tier or three. Can you do it while staying at tier one?

technically that is a tier one