Look how Ossir is crushing everyone in duels! And Dungeon has such a powerful effect – doubling the spell power!
I played with him yesterday in the Jubilee event. He's quite mediocre. He doesn't have any dragons. He has few close-combat attacking units. A lot depends on whether the ranged units can attack before they get damaged. He doesn't have many units to fight with.
And Dungeon doesn't have a doubling of spell power. The enhancement is 1.5 times, but it consumes twice as much mana.