I know they used combustion on Amber-Shaper Un'sok when it pretty much eliminated Phase 2 for their kill on that. But do you think this is an exploit? Decide for yourself and post your opinion. What I gathered from Tsulong is that they used a Guardian 4 piece that abused Might of Ursoc to heal the boss for the last 30% of his health in his day phase.
You can see it at both Kripparrian and Six's streams.
Go to the last 10 minutes.
6:56:00 - 7:01:00