I went the
"fast way" It relies on monsters as the chief part of the strategy
I built Goblin Chieftan (SEN) and Cloudburst (RAV) from the ground up. I didn't do any fancy infusing but I did make sure to put Phy resistance +16 on him, just any will do. I'd also rec it for Cloudburst because Gilgamesh can fuck him up- and if you find decent HP+
RAV+RAV+SEN Gilgamesh and right before he is staggered, use Goblin Chieftan's feral link and it will buff your members. (bravery and strength buffs)
Then SAB RAV RAV- manually select Deshell-deprotect-imperil-imperil-imperil- and use Cloudburst's feral link- and it will en-magic plus buffs and complete the buffs you need.
RAVRAVCLOUDBURST to boost his chain gauge- them comcomcom with your best damage dealer (oh pichuu i love you)
Keep a com+med+sen on you when Gilgamesh enhances his power and switch to rav-rav-sen whenever you can, if you can. You cannot survive without a SEN. And the Com will help from letting your chain gauge dying.
Divider is actually easy to predict, just watch for the calm when he isn't shoving swords up your ass and then switch to SENSENSEN
For weapons and accessories I have Serah's ATB boost Indrait, white cape, and that one accessory that boosts rav roles. For Noel I have his chain boosting (Gandiva?), pain dampener, and a brawler's wristband.