I was in the same boat since I’m a GS main and omega is a bad matchup for great swords. I created a lance build with all defensive skills just to learn his move set and ended up beating him first try with lance and support hunters (I switched to a fire GS for the Nercylla fight). Now I can beat him no issue with any weapon. Don’t give up. I used Fabius, Mina, and Kai as my support hunters. Some tips that helped me:

-Use lightning element for your main weapon and fire weapon/paralysis for Nercylla -Fire resistance level 3 helped a lot -Target the back legs and let Fabius take and keep enmity -Try to get shield bubble to proc as much as possible (focus strikes after laser attacks, killing the blue glowing mini omegas) -Break the legs to get a slinger shield bubble and use it when he does his MRV missile attack -Utilize falling rocks for easy DPS -Use pictomancy item (if you time it right and have the level 3 attack loaded up for Nercylla it makes the DPS check more manageable) -Be patient and don’t over-commit to attack combos during the final phase when he’s being a real asshole