I have no idea how to do Byrnshit. My Cloud with a buster sword+ and Luneth RW only does 4k per hit
Since you have buster ad a zantetsuken, I'd suggest this
Cloud buster retaliate blizzard strike r4 attunement 2
Lightning zant or better sword blizzara strike r4 armor break
sazh power breakdown armor breakdown. I have r2, I alternated these basically preemptive materia
Bmage blizzaga r4 blizzara r4 devotion
Wmage curaga and protecga or shellaga light of fayth
Rw is either lunatic or boon, whatever isn't covered by your white Mage
My wmage was garnet, she will get her sb at some point boosting your attack power.
My bmage was vivi, due to his level and default sb, I do have a non xiii rs five star rod
I would do a retaliate every couple turns for the wm to whack on.
This was how I pulled it off with about 25 seconds to spare. The only difference from what I suggested is my cloud used masamune and lightning used the Vegas from the back row, sazh had the free gun.