I keep seeing posts where people say that they are taking 20th level type parties into Bajin. How do you do this? I don't see how there is that much exp available prior to Bajin.

Not a big deal. I have a party level 12 to 14 and you can make easy fight with a monster that brings 30K XP per fight to each member. You will level up prety fast that way until level 20. Go to Tryton and fight the Hogars, they are easy to kill and bring tons of XP.

See XP Table for details.

Interesting...I notice that the ninja needs exactly as much xp as the bishop for level ups. I know the bishop is supposed to be the ultimate spellcaster, but what's so special about the ninja? Is the thrown criticals and thieving skills really worth all that effort?

I think that they just increase the XP as they add things that the class can do. I don't actually think the Nija should be that high on the exp table. They have free thrown crits and also have critical hit/lock pick/pickpocket/stealth/magic. The problem is that they still only get 6 points to stats and 9 points to skills per level. The result is that yes, they have a lot of things they can do, but there is no way they can do them all. It's an interesting class and I have a ninja in one of my two major games, but the addidtional abilities are not worth the extra exp because you just don't get the skill/stat points to support them (IMHO).