Final Fantasy VIII Centra Ruins

In the center of the three continents, there is a large island that looks like it was struck with a meteor. This is the Centra Island. In the center of the largest island, you will come across the Centra Ruins. This is the home of the legendary Odin. It is also home to the Tonberry clan. There are other dangerous enemies lurking inside as well. You need to have levels above, say...30, and higher attacking weapons. At this time, you should get Squall's Lionheart weapon, as well as the other members ultimate weapons. Though getting the other character's ultimate weapons is not a must thing to do.
The first of the two things we will go over here is the hunt for Odin.

Odin

Before entering the ruins, you will need to equip the Enc None ability. There are a lot of random encounters, and Odin only gives you 20 minutes to find and defeat him. You will want all the time you can get for the fight itself. When you enter the ruins, Odin will bid you welcome. Simply go through the areas, until you reach an alter. When you activate the alter, an elevator will take you up. When you reach the top, you will notice two ladders. Climb the one on the left, and enter the machinery room. Activate the blue orb in the center, and head back down. When you reach the bottom, you will notice a new blue orb there. Activate this one, and a new set of stairs will appear. Head up the stairs. Here you climb the small ladder to the left, and go to the demon statue. You will be asked if you want to remove the eye, or leave it. Take the eye, and head up the ladder to the right, and go to the top of the tower. At the top, climb the center of the dome, to the other demon statue. Place the eye in the statue, causing a random code to appear. Memorize the code, (or better yet write it down), And remove both eyes from the statue. Now head back down to the first demon, and place both eyes in this statue. A torch will light up to the right of you. Enter the code you got here, and a door will open below you. Inside the room, you will find Odin.

Boss fight: Odin

As far as battles go, Odin is a cake walk. The only thing to worry about is the amount of time left. Odin will simply sit there, and take anything you can dish out. If you have the time, and the Mug ability, steal the Luck-J Scroll from Odin. There is only three in the game, and it will teach the Luck Junction ability. Odin also carries Triple, which should be drawn and stocked. Just make sure you have the time to do so. This should be an all-out physical fight, because Odin is highly resistant to magic, and it takes to long to summon GF's. This is where Squall's Lionheart Limit comes into play. If you can get Lionheart one time, you end the fight right there. For winning the battle, you will get the Odin Card, and 20 AP. Odin becomes a semi-GF. You cannot summon Odin at will. Rather he will randomly appear at the beginning of a battle, and eliminate your enemies. He will never show in a boss fight though.

Tonberry King

Once you finish off Odin, it is time to go after the Tonberry King. First, you should be sure to have plenty of Full-Life, and Phoenix Downs. Try to get Zell or Rinoa at 100% compatibility with Diablos. This will make fighting Tonberry's a lot easier. Of course you can use my sure fire method, and have Squall in HP critical, with Lionheart. This is rather easy to do, since most of Tonberry's attacks are instant death. When you think you are ready, save outside, and head back in. Go to the top of the first set of stairs, and un-equip the Enc None ability. Stay in this area, and run around for random battles with Tonberry's. You will need to fight at least 20-30 Tonberries, before you can fight the king. When you have defeated enough, the Tonberry King will immediately take its fallen Tonberry's place.

Boss fight: Tonberry King

In the fight with the Tonberry King, Diablos will no longer be a usable weapon. Tonberry King's attacks are about the same as the regular Tonberry attacks. The real change is the attack speed, and it's Junk ability. This enables Tonberry King to drop random objects on your head for massive damage. Have Squall in HP critical, and unleash his limit breaks. Have one of the others summon Doomtrain to cast Vit-0. Other than the junk ability, this is a pretty straight forward fight. Just keep everyone in the game. When you finally defeat him, he will apologize. He will then become a GF with some nicer than normal abilities. This alone is worth the trip to the Centra Ruins.

This is all there is to do in the Centra Ruins. If you are having trouble with the ruins, You might want to go level up first, then try again. It is pretty difficult to defeat the Tonberry King, considering the amount of Tonberries you got to go through. Just be patient, and you will get there.