pokemon super mystery dungeon grind super fast

 
Yes, the difficulty has certainly increased from previous games. (Which is saying something – the Explorers series in particular were known for being pretty tough, but Super Mystery Dungeon is even more brutal.)
IiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIi

Apples are common, so feel free to explore around each floor rather than taking the stairs right away. However, also be aware of your situation. In some cases, it's best to move on as quickly as possible (for example, if you're plowing through Reviver Seeds, or out of any essentials such as Elixirs or Apples).

IiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIiIi
Also, for dungeons that have a break point in the middle, you're able to turn around and go back to town (at no penalty). This allows you to do the first part of a dungeon repeatedly, which can be good for gaining levels and items without wasting as many Apples.
Still, remember that overleveling isn't the only (or even the best) solution. Items and strategy are extremely important in this game, so make sure you're collecting emeras, and always bring at least a few of the following into each dungeon: Apples, (Max) Elixirs, (Tiny) Reviver Seeds, Oran Berries, various Wands