Proto Omega is kinda easy.

He got two forms : four legs, and two legs.

During the four legs mode, he got a physical shield, which means melee can't dmg it. BLM only makes the dmg during this form. Also, you have to kite it during this mode, you can't straight tank.

During the two legs mode, he got a magical shield. This time, melee can do great dmg, and blm's nuke will be reduce by 50%. It's preferable to straight tank this mode, but you ca also kite it.

Now, i'll talk about what is a MUST to do :

1) Do not panic, never.
2) Always keep it DoT : poision, bio, elemental DoT, etc etc
3) Melee, do not engage four legs mode, cause you will hit it for 2-3 dmg.
4) BLM, you must stun it everytime stun is up. Always use stun when it's ready
5) When he changes form, he summons a Gunpod. This gunpod can be really nasty, and make us wipe if we don't kill it asap. The key is, when the gunpod pop, someone will have to voke it, and the BLM have to nuke it asap. It must be killed fast.
6) Noone lot before I tell him/them to. It works for coins, chip and Homam pieces.