I decided to wrap up the 13 bonus battles tonight rather than tomorrow.
Chaos Bahamut was a straight forward, first attempt victory.
Caius was shockingly way smoother than expected. It still took ~ an hour of battling him, but that was all about learning just the right time to use what abilities.
Turn 1 = Shield, Guardian, Protectga, FB, LS.
Turn 2 = Banishing Strike, Shell, MBD, LS, LS.
Delay Wall for a few turns.
Spam LS / Breakdowns / SC / Curaja to build SB bars
Reapply Shield / Guardian
Whip Kick -> SSBs / SC
Reapply Wall
Use remaining LS / BD / SC
Whip Kick -> SSB / Banishing Strike spam and hope for quick death.
Delaying Wall and saving as many Banishing Strikes for the end were the big key. Basch took a hit RIGHT as I was casting my second Wall and Locke went down right after his last SSB cast.
I didn't end up actually using Thief's Revenge, so I could have brought Armor Break instead for slightly higher DPS.
Random Observation
I've been using a Shared Medica relic for all of these bonus battles with my Wall RW, DPS, DPS, Fran, Basch, Garnet team. However, I can barely remember having to actually use it. On the flip side, I use Yuffie's Clear Tranquil frequently in my Shout RW, DPS, DPS, Fran, Yuffie, Garnet teams. That just goes to show how big of an effect stacking mitigation can have.