So, after many trials and tribulations, I managed to beat Omega. Even better, I managed to get Mastery.
My set-up:
Aeris L55 - Faith R2/Stop R5 - Zeal
Thyrus++, Defense Veil+, Emerald Ring
Tyro L65 - Thundaga R3/Swift Bolt R4 - Devotion
Valkyrie, Circlet, Magick Gloves
Rinoa L65 - Ruinga R2/Thundaja R4 - Attunement II
Rising Sun+, Estharian Helmet, Amulet
Hope L52 - Reflect R4/Thundaga R4 - Vow of Vengeance
Skycutter+, Shinra Helmet, Mage's Armlet
Cloud L80 - Thundaga Strike R3/Thundara Strike R3 - Dragoon's Determination
Cobalt Katzbalger+, Adamant Bangle, Silver Glasses
RW: Boon
Weak link of this party: Cloud, hilariously enough.
My opening turn was basically Aeris stop, Tyro Boon, Rinoa Thundaja, Hope Reflect (on Cloud), and Cloud using Thundaga Strike. Then, of course, Flash eventually landed on Cloud through the Silver Glasses, so he just got to stand there and look pretty for over half the fight. Faithed Tyro Swift Bolts and Hope Thundagas were doing about 5.5K each, and Rinoa was doing 7K Thundajas before Faith, and 10K after.
I took a lot of time gambling with Aeris by weaving Stops and Faith uses, while everyone else just spammed damage. Once Tyro/Hope/Rinoa were all Faithed, she only cast Stop.
Then of course, Omega was getting time when he wasn't being Stopped. Landed a Rocket Punch on Cloud, and then a Blaster, which killed him. At the time, I finally decided to say "screw it" and left Cloud for dead while I ran to everyone else to clean up.
...And then the medals screen comes up and lo and behold, I only lost one medal... For Actions Taken.
Go me!