I guess I will try to make a counter argument:
Lux has fought people who can nuke hundreds of meters with an attack like
Mel Gizalut and
Rosa Granhide. Lux has forcefields that can protect him from Stella's attacks, one for his body and the other covers the Drag-Ride, so Stella can most likely blow them, but they regenerate extremely fast.
Stella also has to deal with the versatility of Reload on Fire, Lux can use it for much more than just increase his speed, he can also decrease the power of Stella's attacks by 10 and amplify himself by 10, he will do that if he is pushed (he did it againts Singlen). He can also reduce the heat of Stella's fire by 10.
Empress Dress will protect Stella from Lux's regular attacks, but Lux with Over Limit is more serious than the regular, so he will go faster (if not at the start) with his secret techniques like Recoil Burst, which can bypass 5 forcecields which were potentiated by Reload on Fire (and that was an extremely tired Lux that was almost without energy) or End Action which can negate Low-High Regen and already destroyed an extremely durable elemental barrier alongside 2 other forcefields (and that was Yoruka, who is not as strong nor skilled as Lux).
About the stamina, Lux already fought people who seemed to be incapable of feeling tired like Philuffy, when Lux was already tired she was like if the battle haven't happened and he still managed to won. It's not like Lux have better stamina (especially because this form lasts for 5 minutes), is that Lux already fought people with much more stamina.
She can defeat him with her AP advantage, but hitting him will be the hard part. So there is this.