Wonder Boy in Monster World

Magic is easily found in chests throughout the game, and sometimes enemies drop them when you kill them. All you have to do is use each type of magic at least once.

There are 6 types of magic:
Fire Storm
Thunder
Shield
Quake
Power
Return

NOTE: You can only get 1 Return magic, and it's found in a chest in the underwater labyrinth.

Thanks to Mimming for pointing this out:

To make it a lot easier for you, there is a hidden door to a secret chest in the pyramid which contains one of every type of magic except 'Return' (Fire, Thunder, Quake, Power, Shield).

Stand exactly where I am in the next screenshot and press . There is a secret door that takes you up to the chest: