Your second trip into the Sunken Shrine is much less direct than your first. You will head to 2F from the entrance, but you will find yourself traveling up and down the bottom four floors during your trip. The encounters on each floor do not change even on unconnected sections of that floor. There are few treasures on this route, but you won't want to miss them—particularly the Ribbon.
Sunken Shrine 2F
On your first trip to 2F, you will find yourself on a large and empty floor. There are no monsters here you haven't seen before, and you won't find any WATERS here. There are several rooms on 2F, but they contain nothing of interest. To reach the stairs (which lead to a new part of 3F), you'll want to head as directly north as you can. This will lead you to a tiny floor containing stairs leading up to a new area of 4F.
This floor is fairly small, and consists of a single empty room with a square path surrounding it. You start off in the northwest corner, and the stairs to the third area of 3F are in the opposite corner. The next floor contains a single tall room containing two chests with a total of 560 G. The stairs to the north finally lead to the end of all the up-and-down traveling.
The second area of 2F is similar to the main part of 4F. You'll find a number of paths connecting several rooms containing treasure. The room south of your starting position contains two chests and 15,825 G. The eastern approach to the western chest is guarded by WATERS, which can be avoided by approaching the chest from the south.
The next treasure room is located far to the west, in the southwest corner of the floor. You'll find three chests here, with a total of 5,835 and the Power Gauntlet. This is a disappointing piece of armor that casts SABR on its user. In theory, this is one of the most powerful items in the game, but SABR is bugged and doesn't work, rendering them effectively useless. You'll have to fight your way past monsters to collect this chest, which is in the northeast corner of the room. You'll fight WATERS from the south, or a combination of WzMUMMIES, COCTRICES, MUMMIES, and PERILISKS if you approach from the west. You can safely collect the Gold if you avoid those squares.
North of that room you'll find a third room, this one containing a single chest with a second Light Axe. You'll be attacked regardless of which direction you approach from: the guardians are GrSHARKS and possibly a WzSAHAG to the west, or a combination of SeaTROLLS, SeaSNAKES, and LOBSTERS from the south.
The fourth and final treasure room is east of the one containing the Light Axe. There are four chests here. The western chest contains the Ribbon, the best piece of armor in the game. Unlike in future games where the Ribbon just prevents status ailments, this Ribbon grants resistance to all eight Elements. This means you'll take far less damage from most spells and special attacks, and become virtually immune to status ailments.
The other three chests in this room contain a total of 19,990 G, but there is an encounter point against GrSHARKS and a WzSAHAG diagonally between the three chests. You'll need to fight these enemies twice if you grab the 7,340 G in the northeast chest.
Once you're done collecting treasure, head northwest from the room where you found the Ribbon to find the stairs to 1F.
Sunken Shrine 1F
The bottom floor of the Sunken Shrine is devoid of treasure, and features a sprawling layout. The only new encounter here is with large groups of 3–6 WATERS. They are tough but far from unbeatable. The quickest route to KRAKEN is to head west at the first opportunity, though the north path will eventually get you to your destination as well. Head north at the first intersection, then west again to reach the main part of the floor.
You'll come across a huge room oddly containing a series of smaller rooms. The door to enter the room is to the south, and you'll need to work your way to the northwest. Once outside, continue north and you will quickly come upon the ball that triggers the battle with KRAKEN.
KRAKEN
KRAKEN doesn't have as many tricks as the Fiends you've faced before, but he makes up for it with eight nasty attacks that can cut down lightly-armored Warriors with ease. Fortunately he will instead use INK, an attack that causes Darkness to the party, 1/4 of the time.
Your primary offense against KRAKEN should include FAST used on melee Warriors as usual, but a solid defense is an important component of this battle. FOG2 and INV2 are both effective in blunting KRAKEN's attack. KRAKEN doesn't resist SLOW (or SLO2), so that's worth a shot as well—it's unlikely to stick, but if it does this fight will become trivial. LIT3 is also effective if you have the spell slots left, but don't bother keeping them just for this fight.
As boss fights go, the one with KRAKEN is pretty straightforward. His basic attack can take out a mage, but you will still have a chance unless you are particularly unlucky. Best of all, you can teleport right back outside Onrac once the fight is over. Congratulations, you've restored light to three of the four ORBS!