Long Gui

I remember defeat it using pure paradigm style.

SEN SEN SEN
SYN SYN SYN - starter
RAV RAV RAV
SAB SAB SAB
COM COM COM
MED MED MED

Start the battle with Haste on Fang, switch to SEN SEN SEN to ward the Ultima. Depend on party's HP, you can switch to SAB SAB SAB / MED MED MED. The key is to inflict Slow on Long Gui's head with Fang, afterwards inflict Deshell/Imperial on both it's legs. (Use Librascope if you have time in the beginning of game if this is your 1st battle with Long Gui)

Use RAV RAV RAV to stagger its legs while keep watch of its stomp. Once it stomp, switch to SEN SEN SEN and then immediately switch to COM COM COM, use Highwind to finish the leg. Repeat the strategy for 2nd leg.

Once it fall, switch to MED MED MED, cure the party back to green bar, inflict Imperial on Long Gui, others will inflict Poison, Deprotect, Deshell. Once these 4 debuffs done, switch to SYN SYN SYN, cast Bravera/Faithra + Haste on Fang and team, others will cast Enfire, etc.

Switch to COM COM COM, attack Long Gui for 2-3 full set of queue attacks before unleash Highwind. Once done, switch to SYN SYN SYN, refresh all party member with Protectra, Shellra and Veil. This is an important steps even when Long Gui is still incapacitate at the moment because by refreshing the buffs, you EXTENDS the existing offensive buff on Fang AND PLACED the defensive buff in the front buff queues.

Once Long Gui rise, he'll randomly throw off Ultima + Roar combo, should that happen, switch to SEN SEN SEN, remove Daze with Foul Liquid, if you have done the earlier steps, the ROAR will only remove the defensive buffs leaving offensive buffs untouched.

Repeat the initial strategy again, inflict Slow on Long Gui's head, inflict Deshell/Imperial on its legs, watch out for Stomp (and Ultima + Roar), unleash Highwind to take off the legs. When it fall, repeat the steps and this time go all out on COM COM COM attacks.

This is my head-on strategy, the winning time is not impressive (around 7-9 minutes) but it definitely allows me to take out Long Gui. For weapon, i can just use Tier 1 weapon but accessory wise, i have to rely on fully upgraded one.

FANG
- Pandoran Spear LV1 (Improved Debilitation)
- Kaiser Knuckles x 3 (STR + 300)
- Genji Glove (Uncapped damage)

LIGHTNING
- Axis Blade LV1 (Attack - ATB charge)
- Kaiser Knuckles x 4 (STR + 300)

Vanille
- Belladonna Wand LV1 (Improved Debuffing)
- Magistral Crest x 4 (MAG + 300)


PS - Omega Weapon is Lightning final upgrade weapon's name, you must be referring to Tier 3 weapon term. For others, their weapon name differs for each characters.

Really, there are three keys to an easy defeat of Long Gui:
1. Fang as party leader (Highwind really helps take down the legs)
2. Two other members who have Curasa or Curaja (as opposed to Curaga, which is not % based)
3. Genji Glove on Fang and at least one other character

In my second playthrough, I triggered a battle against one by mistake (thought it looked like just an adamantoise), and I only had tier 2 weapons, and the same paradigms I use for shaolong guis and adamantoises (syn sab sab, sab syn syn, sab rav rav, com com sab, sen sen sen, med med med). I didn't have any issues, and didn't have to use any elixirs or summons. But if you are having some trouble, summoning is a good way to get the legs down. You can use an elixir if needed to get one more summon. The Renew technique (2 TP) also helps if you get in a bind (ex: medics are stunned or ko'd).