The leveling system seems fine at the moment, you don't want everyone to level up too fast and rushing the content.

1. Do your Daily Missions and Activities and redeem the froggies in your Daily Practice.

2. Play all your available Plot Instances and Elite Instances, when you don't have then Sweep it to get Experience. (Specially every Monday which gives x2 exp for sweeping)

3. Participate in Events specially those Events that gives good amount of experience like Nine Tails Invasion, Rescue the Jinchuriki and Etc.

4. Play your Ninja Exams until your power cannot defeat enemies. Ninja Exam gives experience too tho its not too big but atleast, still an experience.

5. Check your Benefit Hall and see if you have available Resource Recovery. (This usually happens when you failed to redeem 1 or more of your Froggies in Daily Practice)

6. Clear Survival Trial, it give experience too if im not mistaken.

7. For your other ninjas, you may opt to level up them by just killing those random ninjas in the fields. But if you are lazy, then you c*e your Buns and Meats.

8. If you are Level 50+, go find a partner and go for a Wanted Mission. Doing it alone is okay but doing it in Team gives Double amount of experience.

If i failed to include other ways then just post it below me.

Nine Tails and Rescue the Jinchuriki also gives xp.

Team Instance also gives exp (A tiny amount)

Cat Quiz also gives some exp