Omega Ruins is the top-level secret dungeon of the game, with a lot of the most powerful standard foes the game as to offer. It's divided into two distinct floors, both with a Weapon boss at the end.
The First Floor
Enemies
First of all, before you take even one step away from the Save Sphere, you need to ask yourself a question. That question is:
"Do I have a character with a weapon with First Strike on it?"
If you do not, either leave now and buy one or customise it yourself using <items>. There is one reason for doing this. The Great Malboro here will 1) always Ambush you and 2) always use Bad Breath. This is near unsurviveable when used normally but when the first turn is always it you have absolutely no warning and can end up with a Game Over before you've even realised it. If you absolutely have to capture Great Malboros it is a very good idea to do so either by having a character with Confuseproof and Berserkproof (or Ribbon) on their armours to save themselves from losing control or to go to Sin and capture them there instead. However, this is not to say that the Great Malboro is the only monster you will come to hate in Omega Ruins. The Demonolith makes another grand appearance, often in pairs this time, the very random and Death-loving Wraith, and there are others which are capable of nasty or numerous status effect attacks.
A lot of the enemies down here are immune to sensors or Scan so any data that has been discerned about those fiends are supplied where appropriate.
Zauruses are lizards with about 8,000 HP. Their defence isn't too shabby which may mean they survive a hit from Tidus; so two hits kills them. They will also inflict Silence and Petrification with their normal attack, so watch out. This is by far the most common encounter on the first floor, especially near the save sphere, in conjunction with... Mimics are initially disguised as treasure chests that you encounter from time to time in any normal battle. However, attempt to Steal from them, and it'll say 'Nothing to steal.' followed by exploding into bits of treasure chest and energy vaguely shaped like any number of random fiends. The shape it changes into determines its stats, but it is most likely armoured and immune to Armour Break. However, killing one of these nets you an excellent 50,000 Gil, which can (and most likely, will) be exploited later on in the game with Gillionaire weapons to increase that to 100,000 Gil per mimic killed. Mimics cannot be captured. Spirits have about 12,000 HP are similar to Larvae in that they absorb some elements and they can cast those on themselves to heal them and raise their Magic stat. However, this version of them has Poison Mist which is a Bad Breath-esque attack. This is pretty nasty and may be used either as a normal attack or counterattack; the normal attack deals about 1200 damage and inflicts Poison, whereas the counterattack version hits for about 700 damage but reliably inflicts Confusion too. Deal with these quickly. Black Elements are the toughest variety of Element, hugely resistant to physical attack and they null all elements. They have three spells they'll use; Berserk, Demi and Flare. This is quite a powerful combination, especially since multiple Black Elements can appear at once and begin to Berserk your party faster than it can be cured. Demi isn't really a problem BUT it can weaken characters enough to be killed in one by Flare, if not already (Flare hurts). Non-elemental magic (Flare and Ultima) or special forms of attack (i.e. Attack Reels, Energy Rain, or Anima's Pain) are the only reliable ways of dealing damage to these guys. If you want to capture them, then there is one simple solution; Silence Attack. Elements are weak to Silence which thankfully renders them completely useless as they have no non-magic attacks. Every turn they get will just immediately progress to yours. Then, Armour Break them and hit them once to kill them. Halmas are armoured moles. They inflict about 2200 damage with their ramming attack but this also reliably inflicts Slow too. Armour Break them or use Triple Foul to hamper their ability to hit you. These aren't too problematic on their own due to their slow speed but are quite deadly when they show up with other enemies (most often a Spirit) Master Tonberries are like the Tonberries of the Cavern of the Stolen Fayth but this time they've got more health (45,000 HP) and they do more damage with their close-range knife attack. Karma still hits for 10x the amount of enemies you've killed. However, a problem arises that the Master Tonberry halves ALL elements, including Holy.