To be honest, although the "Frost Witch" was clearly no match for Lux, getting captured so easily still had a lot to do with plain bad luck.
Though this impostor had never studied Advanced Elemental Theory, she was still one of Lissandra's own disciples. Unlike the Frostguard Priests, she had her own means of countering spells, and her control over ice and snow was much more precise. Most importantly, Lissandra had temporarily granted her a measure of authority.
This feather Lissandra had plucked from Anivia and temporarily given to the impostor came at a heavy cost to use, but its power was still not to be underestimated. If she wanted, she could make the battlefield take whatever shape favored her.
With so many abilities bolstering her, this impostor was undoubtedly at the pinnacle of the Transcendents. Ordinary enemies, even if they outnumbered her, would pose little threat. Lissandra had ordered her to hold Freljord City and focus solely on defense, so she would indeed have been difficult to defeat.
However, in front of Lux, an impostor was still just an impostor.
The most dangerous thing about fighting Lux was her combat style. Lux was a mage, a Spellcaster who had studied advanced magical courses such as Advanced Elemental Theory and the theory of elemental energy control. Yet when it came to combat, she followed the methods of the Ascended.
She could fight by casting spells, but she could also shape a magical weapon and charge straight at you, swinging a greatsword.
This combat style, inherited from the Ascended, was essentially a way to take advantage of the frail bodies of ordinary Spellcasters, using simple physical attacks to disrupt their casting rhythm.
The bodies of those who underwent the Ascension Ritual were quite powerful. Even basic swordsmanship could inflict lethal wounds.