This guide will assume that you have completed the trading mini-game that you need to access the Sea of Time Islet Cave. If you haven't, the main walkthrough goes through it as part of completing the game.
So, go to the Sea of Time Islet. Mind Read the turtle to be taken to the cave entrance, and go inside. Thankfully, there's no complicated dungeon to go through first; just use Teleport on the teleport circle you come across and you will be taken to a previously inaccessible edge of the water. Go through the door, and you'll be in some catacombs.
This is basically one really long tunnel; the monsters in this portion of the cave are, however, considerably stronger. There are two monsters of interest down here: Cruel Dragons and Wonder Birds. Cruel Dragons have a rare weapon drop, the Tisiphone Edge, which is very powerful. Wonder Birds give an enormous amount of experience, so if you want to level up quickly this is the place to do it.
There are five long sections of the tunnel. In the second, there is a rocking statue atop one of the pillars at the side; use Tremor here to evict a Mercury Djinni (you will have to stand a bit above where the statue appears to be for it to work). Beat Serac in a battle and it will join you.
At the very end, you will come out into a larger room with torches. Before approaching the stairs, save and heal up. Then walk forward, and you will be confronted by the Sentinel!
Boss Battle: Sentinel
The important thing to remember about this battle is one simple fact; the Sentinel is immune to Psynergy. Any Psynergy you attempt to use on him will just be dispersed and do nothing. This does not stop you using things like Cool Aura on yourself, however. He has about 8000 HP.
He is also weak to Venus, so Felix or whoever can use the Sol Blade's Megiddo if they have it to continually inflict a lot of damage. Repeatedly using Haures and Judgment will also help. As for the Sentinel's attacks, he has Armor Crush (which inflicts a lot of damage to one person and has a fairly reliable effect of lowering their Defence), Break (which returns everyone's Attack to normal) and a selection of Jupiter, Mercury and Mars Psynergies like Searing Beam, Destruct Ray and Freeze Prism. Also, try cycling defensive djinn, like Flash and Shade, to minimize damage so you can lessen need to heal your party constantly. This takes a long time, but it is very effective.
The order of the day really, then, is to just tear him apart with attack Djinn and then unleash summons on him when the time comes. Remember to stay well-healed, too, since he gets three attacks per turn. And, whatever you do, do not use anything which can cause Psynergy Seals. This leads to the Sentinel using Armor Crush most often three times in a row which really hurts.
Once you beat Sentinel, you can claim Catastrophe! Catastrophe requires 3 Mars and 5 Jupiter Djinn to summon, but it's really powerful and looks cool. Now, Retreat, and leave the Sea of Time Islet by going back across on the turtle (Mind Read him again).