What high-level enemies respawn in Borderlands?
Solution 1:
All enemies respawn when you exit/reenter the game (with the possible exception of the destroyer). Some respawn if you simply leave the area (such as Crawmerax). If you have the Secret Armory of General Knoxx DLC, I would suggest farming crawmerax (best experience, and the only way to reach max level of 69). The added bonus is he also drops a lot of high level loot, including pearlescent weapons.