Flower Fields in Chapter 6 has an enemy that won't show up that often called an Amazee Dazee. If you manage to defeat it, then you'll get lots of EXP and coins. It is very hard to kill though and is hard to find. An easy way to kill it is to:
1. Get the "Lucky Day" badge from Goompapa in Goomba village after doing a REALLY annoying side quest that involves delivering letters to people in the Mushroom Kingdom
2. Get enough Badge Points to equip the badge
3. Try to find power plus badges (Just use a guide or something to find them)
4. Find an amazee dazee

If you kill it, then you'll get about 50 EXP. If you keep trying to find amazee dazees the you should have no problem trying to max out your stats and have no trouble with bowser's castle.

Not a bad start for a guide man. Though I suggest you try to put in some more details (if possible and necessary). Regardless, I look forward to your future guides.

But if these things are rare to find, Wouldn't it be easier to just level up with some other enemy that is easier to find?

I have to nitpick here because this is a terrible guide. I'm not in any way trolling. I'm just being completely honest because if someone new to Paper Mario were to try this strategy they would likely fail as well as wonder how to complete the letter quest by Chapter 6.

First off, the lucky day badge can't be obtained by Chapter 6. It is impossible because one of the last people you need to talk to in order to finish the letter quest you cannot get to until Chapter 7. Secondly, the lucky day badge is completely irrelevant to defeating the enemy. The enemy will ALWAYS run away if you attack it but aren't able to kill it by the end of the first turn. All the lucky day badge does is stop the enemy from attacking Mario at random times. It doesn't put the enemy to sleep, which is absolutely necessary when dealing with this enemy.

Second, what you really need is the lullaby spell, sleep stomp badge, and sleepy sheep. Use lullaby or sleepy sheep first as it does not cause any damage. Therefore if it fails the dazee will not run away. Then once the dazee is asleep use your most powerful attacks to kill it before it can wake up and run away.

Third, an Amazee Dazee is too rare for you to be trying to level up with it. I've played the game several times through and on average see only one per game, two if I'm lucky.

Instead of focusing on a rare enemy try taking a simpler and more practical approach. For example, after you defeat the Goomba King the next place the game sends you towards is Toad Town on the way to Shooting Star Summit. Instead of going straight through Toad Town to the summit, go back to the Goomba Village where you picked up Gombario and fight all the enemies along the way. This includes the area where the veranda fell. You will get enough star points to be able to level up before you even reach the Koopa Bros castle.

Once you have defeated the Koopa bros, go back to Koopa village then back to Koopa Bros castle defeating the enemies along the way. Use the strategy of going back and forth with each boss in the game (except in Dry Dry Desert because the bandits aren't worth fighting) at least once. You will level up easily, but it will take a good bit.

Another good level up tip: When you are at Bowser's castle attacking the cannons: the cannons don't directly attack you. Instead they shoot bullets out that will attack you the next turn. Kill the bullets but let the cannons live so that they can shoot more bullets out and you can then kill in the next turn with no damage taken by you. It is possible to get 100 star points in a single battle using this strategy, but it takes a LONG time.

I'm not trying to discourage you from writing guides. Instead I'm attempting to help you improve them.

I like to use guides when I'm done beating it to find all the little secrets.

rebelyell : I'm the same way. This has always been my favorite game, so it's so much fun to watch people play and read comments about things you never knew about all the times you played it! A lot of them I have found over the years by accident if nothing else, but there are still dozens of things I never thought of. This Amazee Dayzee hack isn't the best because they're so hard to kill and extremely rare, but other like it are great.