Energy
Start missions only when you have full energy or close to it. Each mission will require you to tap the screen to fill the meter with shields. The more shields filled up before time expires, the more your Legend meter will increase. If you can get all the shields before the time expires, you will automatically complete the mission.
Missions
Always help the people at the school to get missions. Complete those missions help become a Legend. During missions, strategize shield gathering so you spend the least amount of energy to fill your meter.