Assume three situations for this, at levels 1, 5, 10, 15, and 20.

1. Straight one-on-one arena combat. A hemisphere with radius 1000' feet, impenetrable by normal means.

2. The same arena as 1, but with a party of Fighter McFighter, Sneaky the Rogue, and Your Friendly Neighbourhood Heal-and-Buffbot Cleric to help out.

3. The two learn of each other's existance at the exact same moment, just before preparing spells for the day. The hunt is on!