It's true we didn't have the full picture fed to us before, only bits and pieces, but Fern is very good at fine mana manipulation and fast.
No wonder Lügner quickly understood he would die before she's depleted in a battle of attrition.
She's so efficient, even with less mana she can outlast him.
And I think she didn't even break a sweat of bat an eyelid. Somehow she's very much like Frieren when fighting.

As for Stark, well the shoulder slash didn't kill him. I guess that was a surprise... A bad one for my tastes. But since he can slash mountains, why not. He's no magician, but he probably still even unconsiously mobilize mana in his fights, like when blocking that slash.