While it is ridiculously annoying that this bug is still ongoing, all you have to do interact with the wall of vines to be taken inside the center chamber. Then, don’t move, but just stand there and used the ranged attacks at your disposal. I received no damage at all from the rock vines or any of the MPs attacks. Just don’t move.
You can autoattack and kill it, but it will probably take about 10 minutes or so. I just did this on my necro (completely by accident) and took no damage at all. I used some condis and other ranged attacks on my scepter, but I did it in about 5-10 minutes. Pain in the butt, but it got done (after dying far too many times and cursing the names of the NPCs).
Good luck everyone!