Just like with the Flame Spirit Shrine, I've found a trick to level up quickly in the Water Spirit Shrine. This trick, however, is a lot faster and easier than the Flame Spirit Shrine's trick. Not only do you get experience more quickly, but there are also three major advantages of this trick. First, you will never destroy the Monster Gate by accident, and neither will your monsters, so you can do the trick as long as you like without stopping. Second, it is impossible for your monsters to get hit, which means you can bring even Lv.1 monsters and not have to worry about them getting KOed. Third, you barely have to move while using this trick, and you don't need to aim your attacks at all, so it requires less concentration. Also, to give you an idea of how efficient this trick is, I'm currently massively overleveled at Lv.62, and I can still get about 2 level-ups per in-game day using this trick while leveling up my monsters. My monsters often get between 6 and 10 level-ups per in-game day. This trick probably nets experience about 5 times as quickly as exploring the Water Spirit Shrine normally (and that is not an exaggeration).
So, here's how the trick works.
1. Make sure you have a high-quality Storm Rod. Optionally, bring equipment that boosts Intelligence, Magic Attack, and Wind Attack to allow you to do more damage more quickly. The higher these stats are, the faster you'll defeat enemies and the faster you'll get experience. The Fleet Foot Rune Ability isn't needed for this, because you won't be moving very much, but it doesn't hurt.
2. Bring 3 monsters with you that you want to level up. These monsters will not be exposed to any danger at all, so you can bring low-leveled monsters without worry.
3. Go to the Water Spirit Shrine and warp to the 4th floor. Equip the Storm Rod and take the teleporter at the end of the hallway. When you warp to the other teleporter, you will notice that your monsters did not follow you. Monsters will not use teleporters on their own, so they will remain in the safe entry hallway as long as you don't call them.
4. Walk forward and stand in between the two staircases on your left and right. A Monster Gate is at the top of the left staircase, and it will spawn 3 Hobgoblins in front of you. Do NOT call your monsters to you. Note that this gate is rarely replaced by a sparkling gate that spawns a Wolf King when destroyed.
5. While standing between the staircases, use the Storm Rod's normal attack. The homing wind blades will automatically hit the Hobgoblins, and you won't need to aim your attacks at all. Every time you defeat a Hobgoblin, another one will immediately spawn to replace it. Because the Monster Gate is high up on the ledge to your left, the Storm Rod's wind blades won't reach it. This means you'll never accidentally destroy the Monster Gate. Periodically back up so you remain between the two staircases. If you move too far forward, the Hobgoblins may start spawning behind you, which is an inconvenience.
6. If you get low on RP, run back and take the teleporter back to the entry hallway. Then, you can safely use foods like Ice Cream to replenish your RP. When it gets late and you're ready to stop training for the day, climb the stairs and destroy the Monster Gate. After you defeat the remaining Hobgoblins, pick up the massive amount of items that will have dropped before teleporting home.