Simplest Answer: Use any half useful Conjuration spell on them, and buff yourself ahead of time. Unless he is doing Sword of the Arcane Order/Mystic Ranger cheese then it should be as simple as that. If he IS doing the above then obviously you must plan based off what sort of tactic he will use.

If he chooses to use a bow then cast Wind Wall immediately, then fling debuffs at him until he is incapacitated. My personal favorites are Grease, Web, and Bestow (Greater) Curse. If he isnt knocked out then summon some Fiendish Whatevers (Gorillas for fun) and watch as he becomes a smear on the floor.

If he intends on spamming Dispel magic then all you need to do is increase your caster level with feats or items so that the check always goes in your favor. If it is high enough level to cast antimagic field? Contingency Dimension Door out of the area of the spell, then plane shift away to the plane of your preference and watch him through a divination of your choice. wait until the field is gone, then cast spells at him till he dies.

These are siiimple ways to deal with him, and obviously there are people here with more imaginative choices. All I have stated (besides greater bestow curse) is from the SRD so screams of cheese should be met with a cheeky grin, preferably with finger pointing