At that stage of the game, my usual tactic is to use:
Ami to stack Big Guard over and over until your max hp cannot increase further. Usually the team HP max can be pushed to 9,000+.
Main healer heals every turns to top the team's hp up.

The other two are the damage dealers.

My tips are:
1. Make sure Ami and the healer are faster than the boss. Ami is faster than the healer.
2. Big Guard HP buff last 3 turns, so when you HP max is enough, Ami can do other things for 2 turns before using Big guard again.

Nobu — Burst DPS + Emergency Healer

Nobu is my hybrid monster. His self‑buff massively boosts all stats, and with multi‑hit relics he crits constantly.

• Triple‑hit setup (Genji + Dragoon gloves)
• Crits ~2 out of 3 hits
• ~30–35k damage per round during burst
• Heals 3500–4000 normally
• Heals 6000+ during his 4‑round buff

He’s the backbone of the team.

Vera — Burst + Magic Resistance

Vera unloads her strongest spells for the first two rounds. On round three, she summons Zoneseeker to give the party magic resistance, which is essential for surviving Omega’s Ultima cycle.

Laka—Hastega + Support

Laka keeps the team alive and fast.

• Hastega at the start or whenever needed
• Emergency Megaelixir when multiple members dip
• Equipped with Phoenix Plume for more HP and stronger healing
• All Phoenix Downs revive at full HP, which is insanely convenient

She’s the team’s tempo controller.

Jasmine—Constant Burst DPS

Jasmine is my round‑to‑round damage engine. Her build is focused entirely on Vigor, crit, and hit rate.

Her gear:

• Behemoth Headgear (+75 Vigor)
• Minerva Bustier (Nullifies all elemental damage)
• Power Sash (Mog Hide & Seek reward)
• Prowess Band (+75 Vigor, +20% crit, +50% hit rate)
• Bone Relic (+35% Vigor)
• Vigor Yuzu (Doubles her Vigor)

This puts her at 1200+ Vigor, and she hits 20k–30k per round with Pummel. She can also emergency revive when needed.

Try to equip yourself with thunder resistance or absorbing gear to make your life easier. Especially on Laka.

Omega Strategy — Round‑by‑Round

🔹 Rounds 1–2: Full Burst

Everyone goes all‑in.

• Jasmine: Pummel
• Vera: Strongest spells
• Nobu: Self‑buff → burst
• Laka: Hastega → heal only if one ally drops below 50%

Goal: push Omega’s HP quickly while staying ahead of his damage.

🔹 Round 3: Prep for Ultima

• Vera: Summon Zoneseeker
• Nobu: Continue burst
• Jasmine: Pummel
• Laka: Heal/support as needed

This round sets up your survival window.

🔹 Round 4: Full Defense

Omega casts Ultima back‑to‑back.

• Everyone defends
• Only Nobu heals if absolutely necessary
• Do NOT attack unless someone is critically low

If you don’t defend here, you get wiped.

🔹 Repeat the Cycle

After surviving Ultima:

1. Burst for three rounds
2. Reapplying Zoneseeker + Heal is necessary with Starlet or Kirin.
3. Defend on the Ultima round
4. Repeat until Omega falls

Behavior Note

Omega may shift to casting Ultima every 3 rounds once it drops below a certain HP threshold. I do not sure about this yet. If you notice the timing change, adjust your rotation accordingly.

I hope this helps!