Fastest Way to Level Up in Yo-kai Watch 2

Part 1

Requirements: Complete Main Story and defeated the final Boss. Have $234.

1. Go to The Fish Place in Uptown Springdale and purchase 73 Mackerels.
2. Take the Mackerels Jungle Hunter and trade them in for 1314 JP.
3. Purchase the Mem-o-Vac from Jungle Hunter for 1300 JP.
4. Go to Shoten Temple in Blossom Heights.
5. Fuse Flushback with Mem-o-Vac to obtain Vacuumory.
6. Go to Old Springdale.
7. Right outside the station, you will find a Baffle Board.
8. Summon Vacuumory to re-battle the final boss Dame Dedtimeto.
9. Defeat the final boss and you'll gain about 5000 EXP.

Part 2

Create 6 Smashibull Soul to purchase increase EXP gain.

Requirements: Rank S Yo-kai Watch and lots of money.

1. Go to Whatta Find in Shopper's Row at night.
2. Purchase Ethereal Water for $880.
3. Fuse Predictabull and Ethereal Water to create Smashibull.
4. Perform Soulcery on Smashibull to obtain Smashibull Soul.

At rank 1 equipping the Smashibull Soul gives 2% extra EXP gain from all sources. At rank 10 the Smashibull Soul gives 20% extra EXP gain.

You can equip 6 Smashibull Soul at a time for a total of 120% more EXP.

Part 3

Bring the Yo-kai Draggie to the final boss fight gives your party more EXP.

All of this turns 5000 EXP to about 12,000 EXP.