I imagine you mean Kela de Thaam? She is a rough boss fight, but drops some nice mods at least.
I'm actually not sure...it's the final step of the quest The New Stange where you have to defend against 5 waves and then synthesize the enemy Warframe. Almost finished it solo after I posted but still failed. Got a few tries with a random and failed those too. Finishing it is a prerequisite for a few things I believe, so I kind of have to tough it out.
Side note, for a while I was feeling pretty powerful, but now I feel super anemic. Only Exaclibur's 4th ability is still pulling it's weight in terms of raw damage. Radial blind is good too but I can clear more of the waves with the sword thing. I've spend most of the day working on additional weapons and boosting mods too, but I feel like I may have over extended.