PSA: The heat haze trial b1f seems really good for EXP farming.
The weak ninjas give ~4.3k exp per ninja and have 800~1k hp so you can kill them in 2~3 La spell. All you need to do is to survive their initial attacks.
The red circled spot has a guaranteed spawn of 2~3 ninjas for a total of 8~12k exp per spawn, better than the usual exp farm spots. You can just kill them, exit, and come back.
Just don't fight the sentries, those ninjas have way more HP but give the same amount of exp.
This does mean you have to reset the quest and fight the ninja rape gang again after you unlock the ninja class though.
Also the loot is pretty much nothing.