The key to Azys Lla

[Enkidoh]

Something to remember also, Igeroyhm spears the Warrior of Light with dark Ascian magic that practically immobilizes them (notice how after she drops and releases them, the WoL is panting and kneeling in pain on the ground) so little wonder she and Thordan were able to get the key.

It is also justified that at that point the WoL hasn't completely restored the Blessing and the Crystals of Light either so they're not quite up to full strength thus allowing an Ascian to overcome them (Igeroyhm even remarks that the WoL's strength is returning but is still not enough to overcome her power). By the time the WoL reaches the Singularity Reactor in Azys Lla though, it's a different story - the WoL completely trounces both her and Lahabrea and... well.. we know how the rest turns out.

So yeah, although it can be chalked up to "she couldn't be defeated until the plot said she could be defeated", it is fully justified for the aforementioned reason the Warrior of Light's Blessing has not been completely restored with all Crystals relit - once the final Crystal is relit after meeting Tiamat though, they're more than ready and able to curbstomp both Ascians at once even when amalgamated as Ascian Prime.