Cotton 100%

This achievement (and all the Beat X ones) is awarded for completing the Standard mode in the game. Standard mode allows saving at any point and also rewinding the game any number of times by holding the button so anyone can complete it with ease. To make it even easier, select the config menu in the game (not in the main menu) and press the button. Set the difficulty to Easy and the lives to 9, though you should never die with the rewind function. You can beat almost all bosses and enemies with ease but you will want to keep 5 of your magic attacks for the final boss (not the your clone witch but the huge fairy with the shield). I would suggest to choose a magic set with the fire breath. You can regain magic by killing the laser frog like enemies, you will find a handful on the last level so feel free to use some magic on earlier bosses (really only the Ent enemy needs some). As soon as the final battle starts spam the fire breath. Landing five of those kills the final boss instantly. If you don't have enough magic then you are looking forward to a long and drawn out battle where you'll need to get behind the boss when she does her pillar attack from too close.
Shoot (infinite)
Bomb (infinite)
Use Magic (3+how many you can find from blue bubbles)
Rotate between the three Magic attacks you have

After completing the game for the first time, you will unlock a bunch of other cheats like invincibility or infinite magic which could help a bit with the point challenges in this mode.