Xenosaga Episode III: Also Sprach Zarathustra Tips

Optional bosses Omega ID
Omega ID
Location: CIT testing grounds
Head for the CIT testing grounds trough EVS, board the mono-carrier and proceed north through the area past the soldiers. Board the #2 Transporter. Go up the escalator and head through the demonstration area to the eastern transport dock. Board Transport #6 to go to the Special Weapon Development Division. Use the key you obtained from Citrine to open the lock and head inside. Operate the console in front of Omega to start the battle.

Before going with the battle recommendations, I-ll write the previous battle "preparatives":
First make sure you already have the strongest E.S. equipment in the game equipped. Second have a decent stock of E.S. healing items. Third any close range E.S. MUST have D-KILL R and D-KILL C equipped. Fourth have at least one E.S. with Sensor. And finally make sure every E.S. has D-Nullify Effect equipped.

You-ll fight Omega Universitas first, don-t worry he-s just a distraction, eliminate him without wasting any anima at all.
After defeating it, he-ll transform into Omega ID:
Omega ID has a changing elemental immunity ability. It will be weak to a certain element and nullify another. This will change EVERY round. This is why it is important ot have a sensor item equipped. The best thing to do is use the E.S. that can't do damage to heal then switch as the immunity switches. The same goes with special attacks, use them when he is weak to that certain element.
After a while, Omega ID will start using Destruction Wave, which will damage all your E.S.s and inflict status effects(this is why I told you to have D-Nullify Effect equipped). Be sure to keep your HP well above 40000 just in case it manages to critical on one of your E.S.s. Heal up right after, in case he decides to follow up with another one.
At about 60% of its HP, Omega ID will start using Waves of the Berserker. When you see this, have every character Charge, as it will follow with Dark Lord,which can hit your E.S. for more than 60000 HP if unguarded.
After you get him down to around 80% of his max health it will start analyzing your anima gauges. If more than one of your E.S. has level one or above of the Anima gauge there is a chance of Omega ID healing himself for 150,000 HP by using Will of Yin and Yang. So from this point on use Anima awakening level 1 only. Also it will use Dark Lord twice in a row on the same E.S., so be careful about that.

Be sure to get a Finish Strike bonus and good luck with it.
After the fight you will obtain Decoder 06, as well as the Emperor's Bracelet.