There are four wall switches on that floor, assuming you haven't touched any yet switch these in this order:
1st: Southwest (to open the northwest spike)
2nd: Northwest (to open the Southeast spike)
3rd: Southeast
4th: Southwest again (this together with the southeast will drain the water)
The Northeast one opens/closes the second set of spikes on the floor above and you don't have to touch it.
The level is randomized each time but the basic logic is -
1. 4 switches, 3 red 1 yellow
2. yellow needs to be down to open the spikes on the upper floor and always is anyway so don't need to touch it
3. 2 of the red switches need to be down, 1 red switch needs to be up for the water to disappear. The one that needs to be up is usually the one that opens access to another red switch so you have to push it down temporarily and get it up again later
Other useful things I found on my playthrough:
Harpis card doubles EXP gain, aerith gives 20% extra, neither effect is reflected in the post battle dialogue but they work.
Fully upgraded slaves sword is slightly better than fully upgraded best sword you can buy from the store (+5 attack). 4th repeatable request boss drops the item needed for upgrade pretty often.
Holy Arrow spell is better than any sword or sword skill, can get it randomly from a grimoire, you can find 1 grimoire every time you do the 4th repeatable request, all other spells i got from there are pretty much useless
Level is capped at 30 and easy to reach with harpis