I'm moving a duel discussion here from another thread.
Well, if it is a duel and not "Who benefits a party vs monsters more" then i would definitive go wizard.
Either a diviner (who doesn´t like to always have a ini of 30+dex+feats+traits) or a conjurer with his pet (either a Astral Deva or a Nalfeshnee). If i go with the conjurer my pet would hunt the summoner while i finish the Eidolon.
As a diviner i assume i go first. As conjurer it´s a matter of "who rolls higher wins" (as is level 20 most of the time).
Lets start the fight with a timestop, followed by a Prismatic Sphere, and then i gate in either an Pit Fiend or an Star Archon helper (which one depends on my alignment). After that i ready an action to cast Mage's Disjunction if the eidolon comes into range and follow it up with an dominate monster spell. As his Eidolon is not killed the summoner can not summon a new one and the hunt may begin...
On second thought, strike the dominate monster and make it an quickened Magic Jar instead. Time to kill the summoner with his own Eidolon...
Ok so to recap, you go first. Your first cast is a timestop rounds.
1st round of time stop: Cast Prismatic Sphere2nd round of time stop: I assume you move out of the prismatic sphere to cast your Gate, otherwise the creature will be stuck in the sphere with you. You cough up 10k gold and on we go. Assume you are neutral, so you summon a star archon for example.
3rd round of time stop: Do you cast magic jar or do you ready to cast mage's disjunction? You can't do both (if you ready, you can't cast other things without losing your readied action, if you cast magic jar, you're disembodied so you can't cast spells)
I'll let you clarify before I propose a response...