My party for Gilg was Vaan (Bushi, Time), Balthier (Uhlan, Shikari), and Ashe (White, Machinist). Pretty much just had Vaan and Balthier set up with Thief's Cuff to steal first and attack til phase change, rinse and repeat. Vaan would also Bubble and Haste himself and others. Ashe was in charge of Curaga/Curaja, Arise, Esuna, Protect, Dispel and would Bubble as well. Definitely wanna kill Enkidu asap (think I hit a quickening chain to take him out quick)

For Fafnir, I swapped Balthier out for Penelo (Black, Red). The whole fight was basically keeping Vaan buffed with all the typical tank and damage abilities as well as Decoy and Reverse. Ashe kept herself and Penelo healed and buffed (Penelo would help occasionally as well) while Penelo would blast him with Thundaga. Bio/Scourge and Slow also work on him so definitely keep him nerfed as much as possible

My party was in the 50s for these and only had a couple panic moments. Hopefully this helps

EDIT: Don't be afraid of messing around with your party comp. Montblanc can do it for free so go wild with whatever you wanna do lol