Comment by Serpent in the Grass on Mushroom Brave - Chapter 82

The train until you drop strategy really works better the weaker you are though. This is because of the stamina issue. If you are lower level your stamina will be very low, so you'd need to spend far less time training until you lost that stamina. In other words, instead of being able to train say only 1-2 times every 24hours, you could lose your stamina after only a few minutes of hardship. This would allow you to squeeze in 4 such stamina workouts every 24 hours.

Additionally leveling is usually a firm of stacking, so the leveling would be based on the stats you have at the moment. So instead of in say strength, going from say 5 attack to 10 when you go from lvl1->lvl2, if you say trained for one year four times a day you would have 4x365=1460. Which would give you a base attack of 1465 at lvl1. Now say it stacks at least the first lvl up. This would give a base attack of 2930 at lvl2.

Well, all is not lost.High leveled characters might not be able to get an initial stacking bonus (if such a thing exists) but if they find a training that can drain their stamina within a few minutes they would still be able to do it four time every 24hours. But even then the low bonus of 1 isn't worth it since the grind takes to long. This however can be remedied if you have stamina potions.

So basically if Duo found a training that took only minutes to drain him (like s** with Piggy), then he could just drink a potion right after. We have already seen him produce stamina potions since he has that ability. So in theory as long as he has enough ingredients to make new potions, he should be able to do the zero an incredible amount of times. Say s** training takes 10 minutes. 10, that's six times an hour. So 6x24=144. So if he had somebody there to feed him an Piggy potions every time they dropped, they could raise their stats really fast.