Hard is not too much harder than Easy, and i would suggest playing on Hard on your first playthrough. After you get the "Big Money!" achievement, I suggest running past every enemy that you dont have to fight. You may need those lives later on. Save your PTE for when you have to kill X amount of enemies to progress, and take your time on the Battles. Dont rush yourself, and learn the moves of both your character and your opponents. Use your PTE against an opponent who has just spawned to set them down another life. A PTE used on an enemy with 0 damage will probably lose a life by the end of the PTE, so dont waste a perfectly good PTE on an enemy who has 200 damage.
This can be unlocked via Level Select after youve beaten the game. Replaying each level on Hard will still unlock you the achievement.