The Keys charts use only 5 keys (C-G on the keyboard). The easiest way to get this achievement, in my opinion, is to play on Medium. Even if the keyboard is completely new to you, you should be able to at least 3* the early songs without much trouble.
Note: This requires a keyboard controller to unlock. You can use a guitar controller to play the Keys charts (available after completing the "Guitar Immortal" goal), but no Keys-related goals or achievements will unlock until you play something using the keyboard (you'll still make progress towards them, however).