It was checkmate.
Lux' Magic was potent and versatile, but for her to match the kind of power that Sebas was unleashing, she had to pour all of her Mana to restrain him—albeit barely.
As a result of that, she had no proper defense in place around her.
With Britta's swift motion, and the deadly power she wielded with the blade, Lux's survival without a defensive barrier was zero.
Britta was an Elite Swordsman, which made her practically invincible to most when it came to the blade. Her Skills her buffed her physical state and attacks to an absurd degree. Plus, since she was a Heroic Rank Adventurer, her Stat were not to be discounted.
If anyone could kill Lux as she was now, it would be her.
However, this wasn't the only reason why the current situation was a deadlock.