Did Ultear know there was a different way to defeat master Hades?
Urtear did know for sure. She had been trying to deceive Gray all along. She told Gray that the Iced Shell was the only way because the spell traps the caster as a shell of ice for all eternity. She wanted him to die because she thought Gray was the one responsible for her mother not loving her which was not true at all.

Urtear knew more about Hades than any of the outsiders. She obviously had a plan to kill Hades herself but having Gray kill him would be killing two birds with a single stone.

But Gray realizes that after he over hears her and sees her trying to kill Juvia.