I'm approaching the stat cap. About 8k away. would anyone know what level of speed I should rest at? I think 4k is minimum. here's his stats:

Level: 4671 Health: 8601 / 8601 Strength: 6565 Defense: 5837 Speed: 5363 Intelligence: 9494 AP: 593 Tier: 13

oh also is there a list for what items unlock challengers?

I'd generally advise you to go to 6001 so you're above Prodigy's base speed. Almost no one else in the colosseum is as fast(Warlgthroth) or faster than that(The Mage has 7k, Hydragellos 7 or 8k IIRC, and the Super duo 8.5k and 9k respectively) and you should still be able to crit your opponents fairly often if you have each of your weapons modded with one critical strike tear crystal.

Edit: As for unlockable challengers, see here.

cheers :) yep all my weapons have the crit strike on 'em.