Erm...

You can actually level fairly quickly up to around 20 or so. I call the farming period between level 20-35-ish the "dead zone" because that's honestly the hardest time to farm. You're forced to farm enemies that are much lower level than you because the ones that give you good EXP all aggro on you, and it's overall just a humongous pain. The place that's best for farming at that level range is also the place that no one even bothers going to anymore and is the farthest from any form of civilization: you have no shop and it's unbelievably far from Mushroom Town.

I'd say that farm goombas until level 3 and rush only stat points into attack, then as soon as possible buy your first weapon from the item store. At this point you should have somewhere around 4-5 attack, and then you can kill lakitus using the strategy shown in my camping/farming video, and each lakitu gives 8 EXP when you only need 70 or so to get to level 4. After you get to level 5 or so and only rush attack then you can start farming gloombas in the sewer using camping strategies until you reach around level 9 or so, and gloombas give 17 EXP and you need 250-ish to get to level 8 or 9. If you do it right you can actually reach level 10 in an hour or so. Just rush only attack until level 7/8, then you can slowly invest in defense, but you should still rely on equipment for most of your defense.