Now that I have the objectives done, I've been trying out different champions to see which ones work against Nebula. One of the best ones I've found is Adam Warlock. He absolutely smashes the fight since he's almost constantly unblockable.
There are some lulls in the fight when the Sovereignty counter ends and takes his unblockable buff with it (and at one time I accidentally block an attack, which also ends the counter and the unblockable), but overall this is one of the smoothest fights I've had against Nebula.
I just figured I'd share, in case he's useful later on in WoW.
https://www.youtube.com/watch?v=X8LTuNPM2PY