What is the best and quickest way to level up my character

Cave of Ordeals 3 and the last version of the Demonhall Mirror which is at the end of the Cave of Ordeals.

I forgot to mention, but getting Statician specialists greatly improve the rate (1% per statician level)....I hear the cap is 1900, but I've also heard that it is 300. Not sure which though....

But in any case, it makes a big difference. There is an exploit in the PSP version which allows you to get staticians for free....I think I posted it in the "Glitches" topic, if you're interested?

You can't get any more boosted exp from more than 300 Statistician, but it might be possible that the level cap for it is 1900, wherever that number comes from...

Where to level up besides cave of ordeal, i mean you have to level up first before wing slayered those demons

The best main story map that I can think of, would have to be the first Celestia Map. It has 100%+ (or at least 50%+) boost to EXP, and 3 enemies on the map (in the 80's, 90's, or around 100, I can't remember)....