I wasn't terribly satisfied with my initial run, so here's another one. This one does require a well built Kain support. I had a Kain support with UW and 30FL. That being said, this is a good strategy for those that decided to skip pulling for Kain.
Timestamp
Party setup at 9:26
My Penelo only has 5000 enhancement points invested into her Force level to bring her to FL10
Raines call is mandatory
Seymour call can be replaced with virtually any call that lands a nice debuff on the bosses.
Cait Sith call can also be replaced by any other call that boosts party HP damage.
0:00 Setup
Reset until Garnet goes first. Use BT+ on her Free turn. Use Raines BASE call to pull Penelo back. Use Peridot Thunderflash to end her turn. In my run I used AA but on hindsight I think it's better not to.
Penelo spams S1 and S2. These are free instant turn skills that will charge the force gauge. When the bosses recover BRV use her call ability to BREAK them. Eventually Ifrit summon will fully charge as well. Don't forget that Penelo can recover skills via AA.
When she is dry, switch her out for Kain, use AA then BT+
Cissnei will get a turn, use her AA, her call abilities then pop summon.
4:10 Summon Mode
Cissnei uses LD
Garnet uses LD
Kain uses LD, the force gauge should be at 100%
Kain uses FR
Kain uses Jump
Cissnei uses LD
You'll come out of summon mode on Garnets Turn. Use Raines LDCA then pop BT mode.
6:36 BT Mode
If done correctly you should have 2 overhead attacks from Garnet plus her BT Expiry attack triggering at the end of her BT mode for massive damage.