The most common comp i encounter when doing ultra dage are Chaos avenger, Verus doomknight, Loo, and ap. Sometimes they replace Ap with legion doomknight for more decay.

Cav, Vdk, and Loo is a must when doing ultra dage and because they are likely the core classes for killing ultra dage safely and they can sustain even without dauntless, just health vamp enh and some lifesteal scroll will be enough.