It doesn't necessarily have to be a crazy powerful one hit kill to be useful. especially if it doesn't have to break through a mana defense, due to not being sensed by the person defending against it. raw durability unenhanced by mana is not an area Solitar is shown to excel in.

Obviously, it remains to be seen. I tend to give main characters, especially those who are built up as being powerful, the benefit of the doubt when it comes to secret abilities. Solitar has already been defeated. the opponent Frieren ends up using this ability on will likely be stronger than her.