Ninja exam is often overlooked. If you try a lot of different setups you might break through another 5 lvls with a bit of luck involved aswell. It might only be 5-10 k exp but thats guaranteed exp you get every day. It will also help you gain actual battle experience (as in learning to counter different ninjas, testing new setups and combinations and finding flaws and weakness in ninjas you face)

Just to give an example: I am currently stuck at lvl 80 ninja exam at lvl 57 while most people around this lvl are stuck around lvl 70 ninja exam or lower. With maybe a few rare ones at 75. This means I get more exp than them every single day and this quickly stacks up to a considerable amount of exp.

Also buy stamina pots from the arena shop on sundays. Thats 2 stamina pots (50 stamina ones) for a bit of arena currency and use them on mondays when sweeps give bonus exp.