Are there any easy and quick ways to level up? My hero and partner have evolved once, but they are only in their mid-20's. It'll take forever to reach level 36.
There is a way.
what you need:
Investigation Brigade Rank Gold
A strong Pokemon (or several) who knows Heat Wave or Sacred Fire (e.g. Charizard or Entei)
(Optional) Mist Ringlet (obtained from beating Mewtwo) which lowers your PP usage
If you've got the basic necessities, go to the Sand Continent and enter the Mysterious Geoglyph dungeon. It's 8F, difficulty: super hard, probably third from the bottom of the list. If you've already been here, you should know it's full of Genesects.
Go to the last floor.
What's neat about this is that the entire floor is actually just one room, which means you can use a room-wide attack to kill all of the Genesects at once.
Each Genesect should give 125 exp and there should be 5 Genesects each time they respawn. If you wipe them all out at once, you'll get 625 exp. (If you bring a super motivated Unown, you'll get 1250 exp).
Additionally, remember that you can skip turns by pressing A+B. Incidentally, if you set the room-attacking Pokemon to be an NPC, they'll attack as soon as the Genesects spawn, which means you can just hold A+B and only worry about your Belly and your PP.
If you do this for a looong time, you'll get a message that the wind is starting to blow. If you keep going, this message will pop up two more times (making three). The fourth time this message pops up, you'll be swept away by the wind and the excursion will FAIL (meaning you have to restart and, presumably, all of your grinding will be lost).
Bring lots of apples and PP ethers (using the Mist Ringlet is obviously helpful here), but you don't have to worry about bringing a ton of big apples or PP Maxes because the "wind blowing" message will come long before you run out.