One build that is worth considering is the "defensive" minded anti-magic build.
Basically, you have a fighter type, an item that can generate an anti-magic field 2ce per day, and lots of adamantine.
Idea is that the best offensive is a good defense.
Spellcasters are hosed (very likely). Magic-heavy builds are hosed. Magic attacks are hosed. Etc. Either rush in and grapple and kill the opponent, or shoot them with magic arrows (which turn magic again after leaving your anti-magic field).
Not very clever, and relatively obvious, but something to consider.
It won't work all the time, but its refreshingly simple to build, easy to play, and somewhat difficult to circumvent.
If a ring of anti-magic field won't work, will a ring of spell storing (major)? Is anti-magic field 5th level for some obscure class? Is there another way to get one somehow? I'm sure somebody can figure it out. A maxxed out "Use Magic Device" fighter PC with a few 6th levels scrolls? You just need to hit DC20 or 25, can't remember...that's another way to go. Just make sure to get initiative, or if there is a round to buff, use it.
Don't forget to sunder all the enemy weapons that become mundane if they step into melee range.
Give the anti-magic guy lots of hitpoints, a spiked chain, a compsite long bow (mighty), grappling feats, etc. Monk might work too in this case, if you can get around the ranged attack limitation.
If everyone has a round to buff, activate it in the round and walk into the arena.
How much would a ring of anti-magic cost, anyway? Obviously that's all you can spend your money on, that and adamantine.
