Ulrik the Undefeated is the Stone Summit hero who, as his name implies, has never been defeated. Defeating him is the objective of the Mesmer-only quest, The Stone Summit Champion.
Ulrik has access to a number of damage-mitigating spells and frequently uses them, so the best method to defeat him is health degeneration from conditions or hexes or spells which ignore armor.
Because of his Stone Dagger spamming, combined with his constant use of defensive spells, Backfire is perfect.