Well, from my experience it's not too difficult for a Sieg to do a lot of damage to her. Basically ...
run in and attack as much as possible, melee is fine. She will always fly high into the air before crashing down and transforming you. Just stop using melee then. Assuming you have at least 1-2 Eirs they should be able to fake res you if you get caught in the spin, but if I remember right there's something else she does right before that attack as well so you should have time to get away.
second form, main thing is that whoever has aggro should try to run to a corner/wall and just concentrate mainly on dodging ... if she's not targeting you at the moment, it's very easy to just stand behind her and totally safe to just unload damage. Again, just like first form, she will counter and fly before transform, and there's plenty of time to get out of range of the lightning.
I do agree that it's much easier to survive the boss the fewer people that are left alive though ... if the whole party is alive it's very easy to mistake who is being targeted and get caught by some random attack which does very high damage.
"It is not a disgrace to fail. Failing is one of the greatest arts in the world," however --
"Believe and act as if it is impossible to fail." ~ Charles Kettering