I *think* I finished at Level 25. Finished the game with 45 HP, 45 FP, and 27 BP. If you're at Level 21, you'll definitely be at least Level 25 by the time you finish. I was able to get through the endgame on my first try, but it was defintely a challenge. I'd recommend having at least 40 HP before hitting the very last areas.
Those stats are pretty close to where I'm at now. I think my HP/FP are both at 40 with an additonal 5 points from badges.
I've been very happy with the level of challenge provided by the game. I've made it through many of the boss encounters just by the skin of my teeth. Hell I died on a tough but non-boss enemy when I finished for the evening yesterday (though it is rare to die on a non-boss encounter).