There is no need for any sort of grinding, especially not on normal difficulty. If you want to do fights without becoming overleveled, just fight one of each new enemy you see, avoiding other encounters. If you struggle with a boss, it's almost certainly due more to your approach/build/equipment than your level.
(The above is true for most JRPGs, despite common misconceptions. It's just that people have it hardwired in their brain to grind when facing any adversity, preferring brute force rather than adjusting their strategy.)
If you ever do want to "grind" levels for whatever reason, it can be done super fast in DQ via metal slimes, so don't worry about falling behind when avoiding encounters.