Slowing it down is essential, so yeet Spider Silk asap to apply Slow. Keep Protect, Shell, and Haste up as much as possible, recasting after it clears them. Item cures are faster than magic cures.

Meltdown is a pain in the ass. IIRC, it deals Omega's current HP as damage, so there's no reduction or negation to mitigate beyond just smashing its health as low as possible as fast as possible. Bahamut does heaps of damage, Meteor is great, and Edge can throw trout without worrying about counters, which can ensure that he keeps his health pretty high. Omega is weak the Lightning but will counter Thundaga; can be worthwhile. Some of the better beatsticks are Holy type, which Omega resists, so switch out if you're using Excalibur.

Other than that, levelling up to get higher Max HP is the key.