No, it's not possible to defeat all the heroes of the Stronghold with Sinithar (one-on-one, that is)! He dominates in duels against Dungeon!
And in Sylvan, Ossir and Anwen defeat all three heroes!
By the way, I played Ultimate with Prizrak, Dungeon vs. Orcs! He beat me. Letos against Goshak! The moral of the story: it's not advantageous for Dungeon to play against Stronghold in Ultimate, but a rush for Vayshana is deadly! Proven!
And dueling Orcs... ugh, they're very weak!