best way to lvl up REALLY fast is a combo of all these things
*double exp days
*10X exp pots
* exp bursting

a lot of people say that they get hugh amounts of exp when they burst
this gets higher when u use the bursting on a double exp day and use a 10X experience potion this will get u 20 times the burst amount

example wrote:

* normal bursting: + 20.000 exp
* double exp day, bursting: 40.000 exp
* double exp day, 10X exp pot, bursting 400.000 exp