I won this achievement in the first area (Saw Mill) on challenge #6 (this one sits in an open area between the red/yellow log truck and the white building). The challenge is just get 3 flourishes (of any level) so DON'T GET ANY FLOURISHES. You want to make only "regular deliveries" by tapping A button rapidly. You don't have to wait for the burger to turn red or anything, just make a quick delivery and move on without being seen. Once you get your chain up to about 18 or 19, you're ready. Find an easy target with the burger overhead (I like the guy chopping the tree, because he doesn't ever see you). Once your target's burger is red and BLINKING, get as close as possible and get a lvl 3 flourish. I did it when my chain was at 18, and got about 108,000 pts from that delivery. If you weren't close enough or get a lvl 2, you'll still be able to get 2 more flourishes on other people before the challenge ends. There is no time limit, so don't rush. If you're having trouble getting enough points, then work on making your chain higher before you start trying for your flourishes.
One last thing to note, the achievement won't pop until you finish the challenge!