Basically, though, we could have won, except that we didn't have any way to fight the vines that the tree could see. I remember, from previous non-failed respecs, that being a whole five or six vines, with rocks and branches and things to hide behind to get the others... but for us it was 31. Almost half the room was in the clear view of the tree, with only scattered bits of cover big enough for one person and out of range of most of the vines.
There was no way for us to kill 31 vines while being three-shotted (the first shot usually being an immobilize) by the still-untouchable ('invalid target') AV. We got lucky once, with the tree mostly targetting the brute (who could live and be healed) at first, and got it down to 3, but around 12 the corruptors were all dead and the last few of us couldn't take out a dozen before they started respawning and the tree killed us.
The main difference between this time and previous respecs was that we didn't have any masterminds. Masterminds have lots of disposable targets for the tree to shoot at, wasting its time. If we'd had a mastermind, we probably could have done it.
Hell, if the tree was 'unable to be damaged' like the mission says instead of 'completely unable to be targeted in any way' we could have done it, 'cause I could have slept it and the corruptors could have debuffed it.
Anyway, after that one time where we got it down to three, we never got close (as in 'below 20') again. After another hour of repeated failed attempts, the leader insisted that next time we'd definately manage it, but anyone who wanted to leave was welcome. Which is when I quit, since I don't really need a respec anyway.
Ever since I7 I've been in a lot of failed teams -- it looks like the devs went out of their way to make the hardest parts of the game much harder. I do not approve.