Shame on me, I can't solo the boss with my weak rogue.
But if you have a decent rogue, here is what you could do... It's only a guess of course.
You loose the stacked buff the second you leave the instance. But you have to leave the instance in order to reset it and get the buff once more. Well what if you could reset the instance without leaving it then ?
In theory, you would be able to get the buff as much as you would like to and only one would go away when you leave the instance for good right ?
And well, there is a very simple way to reset an instance without leaving it.
1. Group with a friend
2. Do whatever you have to do
3. ALT+F4
4. Ask your friend to kick you out of the team while disconnected
5. Reconnect : you are in a fresh instance, but never left it.
There is nothing sure about all this, but my guess is that it should work ! If anybody can try that, I'd like to know the results
Edit : Ho and by the way, to answer Gravity's question, you can use that technique to get the key, reset without leaving and do the rest. You don't need a rogue then ! And i'm sure of this part as I actually did it. I killed Mol'dar, reseted the instance that way, and then I was in a fresh instance, with Mol'dar still alive, and the key in my inventory !