The Sunken Shrine is really two dungeons in one. The eastern stairs from the starting floor (3F) lead upwards to the 5Ftreasure room where you will find some nice items and the Rosetta Stone. The western stairs eventually lead down to 1F and Kraken. I recommend completing the upper path first, since the rewards are great and it is both shorter and easier. The Warp2spell will put you directly outside Onlak, where you can use the Inn to rest up before returning to the Shrine.
Almost every enemy in the Sunken Shrine is new, and with the exceptions of the Aquos and Ghost enemies, they are all vulnerable to Lightning. You will run into a lot more large groups of moderate enemies than the smaller groups of beefy physical attackers you've seen in other dungeons, and there is a refreshing lack of unfair encounters (though you should be careful with the Ghosts).
The treasures found here are the best yet, including a full suit of Diamond armor for your Knight, a number of spellcasting weapons, and the incomparable Giant's Gloves. You will also find the first of three Ribbons here, which will make a character virtually immune to special attacks and spells.
Kraken is the first Fiend you've fought that doesn't have a lot of tricks. He can inflict Darkness on the party with Ink, but the real danger comes from his eight powerful attacks. If you can manage to Slow him, the fight will be quite a bit easier.
The Sunken Shrine is really two dungeons in one. The eastern stairs from the starting floor (3F) lead upwards to the 5Ftreasure room where you will find some nice items and the SLAB. The western stairs eventually lead down to 1F and KRAKEN. I recommend completing the upper path first, since the rewards are great and it is both shorter and easier. The EXITspell will put you directly outside Onrac, where you can use the Inn to rest up before returning to the Shrine.