To use "Time Travel Past" press . The longer you hold it down, the further time will rewind. 4.2 seconds is the maximum amount of time you can rewind. You can see the amount of time travel time you currently have by looking in the bottom left of your screen. The time will increase when your time travel power is not in use. It can also be increased by picking up power-ups which have two arrows on them, like the achievement picture.
To get this achievement, you need to save up your time travel time and get a 4x multiplier. A 4x multiplier is achieved by killing enemies and not allowing any to pass by you. Your multiplier is shown at the top of the screen. When you have a 4x multiplier and 4.2 seconds of time travel time, wait until there are plenty of enemies on screen, then quickly kill them and rewind time. Allow your time shadow to kill all the enemies and the achievement should unlock.
Note: You're very unlikely to get this in the first couple of levels, as there are not enough enemies. Stage 1-4 is when you should start trying for it.