Really easy and you will probably get this without trying in the later levels.
Here a little tip if you wnat to get it early on. Wait till level 1-2 and keep and eye out for a cluster of many enemies near the end before the boss.
Here, you want to do a quick shockwave. Go into the middle of the screen and wait till the first enemy reaches the left side of the screen so all of them are visible at the same time. Now go back to the left side and press LT to rewind time, while still holding the stick to the left side. Release it, when you are in the middle and start shooting your shadow.
After 8 hits, the shockwave will accure, killing all enemies on screen. Now fly into the bright circle and stay there till the end of the rewind to get a perfect bonus. This should net you 15k+
If you don't get it, don't worry. You WILL get it in later levels just trying to survive. Believe me ;P