I (foolishly) agreed to help an MR 10 clan member (I'm also MR 10) with the boss on Sedna. Now, I had never run this boss before, so I took my best gear, and expected a tough fight. For the first and second time through we picked up a pair of randoms, and won without too much trouble despite most of us having no idea what we were doing (or at least it seemed that way). The third time through both randoms left, and we went in alone.
Turns out the clan guy was practically useless. He did basically no damage, leaving it all to me, and he didn't seem to be able to avoid damage from the boss either even though he was playing a Limbo (Limbo has a really easy move than basically makes him invulnerable).
I'm pretty darn tanky in my Rhino, but my DPS sucks. Still, I managed to carry us to the last stage of the battle before losing my last revive. I really need a better weapon or something, but even so we could have won without too much trouble if he had either A) Concentrated on staying alive and reviving me or B) Made sure to bring a weapon with decent DPS and do hit-and-run strikes on the boss.
Anyway, so far the Sedna boss is by far my least favorite, since you need to do some special missions first to get "points" needed to pay for access to the boss fight level. It makes farming her part drops very slow.