The Achievement reads the following: "Destroy all three dragon robots."
They spawn if you can keep your wanted level at 5 for a few minutes. The second and third dragon spawn several minutes after the first one and are colored in blue and yellow.
I got this doing the following way:
1. Set my difficullity settings to "Immortal" because I am wimsy :D.
2. Maxed my size with Resize-Power + Gorilla-Augment (closer to the ground for easier descruction).
3. Changed to Electro-Power and Destroyed everything in the streets with my hands franticly swinging around and holding the shock-button for additional damage.
4. Ignored the first dragon (do not destroy it!) and keeped on destroying everything untill all three dragons arrived.
5. Quickly destroyed all three dragons - they despawn after a while, be fast!
6. Quit the game, saw the Achievement (didnt pop in the game, maybe because its secret)