Normally all enemies stop giving you star points when you pass a certain level and you're forced to visit a new area to be able to build any levels. However, there is one hidden enemy in the game you can fight repeatedly regardless of your level and will give you over 40 star points, the equivalent of fighting 40 easy enemies.

In the flower fields, take the bottom right hand path and cross the spikes with lakilister. A glowing amayzee dayzee will randomly appear with 1/4 odds. It may run away, so try a dizzy spin attack to daze it before the battle starts and kill it in two turns for massive star points. Travel two screens away and repeat the battle to hit the max level easily.