Brilliant Instability Triumph for MoT - What are the exact requirements?

So just did a full King's Fall, and while we are at Golgorath, I had the taint and dealt the final damage to the boss, but the triumph did not pop up.

Anyone else have this issue?

I guess it is possible that someone else did kill him with a weapon, but it would have been super close if that is the case I was watching but forgot to record it.

So just to clarify, I now assume it has to be the absolute killing damage to kill the boss for it to count?

And if so, for those that got it, is this a solo or team triumph? What I mean is if I try and get a team to do it again, will the whole team get credit, or just the person that kills him, because that would be super hard and random to get done.