The problem with this leveling system is that it doesn't work when you intentionally try to increase your stats.
The system is obviously designed to simulate the results of your experience during an adventure, and it does work very well at that part of the game.
It's when you suddenly are in urgent need of HEAL 5 or something that it becomes tedious, it would've been better if you got inflicted by Poison, Dark and Curse a lot in the 2-3 dungeons right before the Whirlwind, that would allow you to level up that spell without actually grinding.

But the way I usually do it when I want to power-level in the end is that I simply walk through the final dungeon as far as possible (while still making sure I can use EXIT, of course), then keep re-entering and exiting over and over until I can make it all the way to the boss.