it....really isn't. The problems comes from the more you level up, the more difficults it becomes. Fighting goombas at level 1 will help you, but this game is the only one that has 0 exp on weak enemies which imo isn't a good thing, but you kinda have to follow the progress of the game. Don't try to overlevel, this will waste your time and you really need to want to to underlevel.
Unless you actually made the wrong choice on level up. Think twice because bp is awfully important (it's in fact the most important because you can actually beat the game with bp only and some strategy).
I actually realised that I jsut followed the game progress to get these levels.
However, i will say that in TTYD, going from level 25 to 30 in post game is annoying, I farmed the gold bill baster which was fun because stylish!