Chemistry
should be able to get the job done, with a bit of luck.
Take a Chemist with a reflect ring and a mage masher, and a KO'd Berserker otherwise prepared to smash things as effectively as possible.
Reflect ring should prevent Omni from doing much to the chemist leaving you free to do a little buffing. You want haste, double HP, and protect. And to be in the back row of course.
I believe mixing Berserk on Omni will still active his Return counter. So you need to use a Mage Masher and set silence, AND get another turn in before silence wears off and mix berserk. Should be doable with proper timing, doubly so if he slows himself.
Once he's berserked you want to keep yourself alive (he will hit hard) and mix Dark Sigh (eye drops+dragon fang), which I believe is unblockable unlike Dark Gas.
Now you should have it in the bag if you got this far. Revive and buff your berserker (HP and protect in case a hit gets through, haste for more turns) and let them have at it. Defense down mix could also help.
This list has them in the order you can learn them. But some have some limitations.
https://steamcommunity.com/sharedfiles/filedetails/?id=524867626
Death Claw is only available in W1 from the Iron Claw boss. If you miss it there you have to wait until the start of W2. And you really don't want to wait to get that.
Magic Hammer is similar, but less essential. It is only available from Byblos, otherwise you have to wait until Drakenvale. Still a useful spell since MP drain is a viable strat against some bosses.
1000 Needles requires a beastmaster to get before W3.
L4 Graviga is available from Ghidras in W1. Missing it there means waiting until the Barrier Tower in W2.
Level 2 Old and Level 3 Flare can be learned at the Barrier Tower if you have a way to cast reflect and a Beastmaster. Otherwise you want to learn L2 Old from Magic Dragons in Exdeath's Castle, and L3 Flare from Tunnelers in the Pyramid (or Exdeath, but it's not worth it there).