The orb starts at 10 count and only -1 on player action. 9 actions to set up is a lot and Machina Garnet can kill the boss via back to back summon and BT mode after that.
Timestamp
Party setup at 5:51
If you don't have Seymour LDCA use your best HP DMG Up debuff call. Laguna, Raijin or even Kurasame are decent options.
If you dont have Leo LDCA use Prishe, Alisaie or Cait sith
If you dont have Raines LDCA the only other option is Onion Knight LDCA
You can opt to use Ifrit summon over chocobo if you need more turns to bring Behemoth closer to 50% but you must be careful NOT to bring it below 50% inside summon mode or you'll get wrecked.
0:00 Setting Up
You just need Garnet to act last.
Machina: AA and BT+
Selphie: Wall, Seymour LDCA and LD
Garnet: Peridot Thunderflash, Raines LDCA targeting Selphie, BT+
Selphie: swap to Ramza and use BT+
Pop summon
2:15 Summon Mode
This is where you complete setup while bringing Behemoth close to 50% HP
Use Machina LD and his LDCA here. Position Machina to get first turn out of summon.
Do NOT bring boss below 50% here.
3:50 Machina BT Mode
I started this at 52% and was just enough to finish the boss off by the end of it.
51