Scrap Metal - 20G (Original Achievement)
To get this Achievement, you must defeat Metal Sonic.
To get to Metal Sonic, you must clear all of Area 1 (Green Hill, Chemical Plant, Sky Sanctuary) and make the Challenge Gates appear. This will cause Metal Sonic to appear above Chemical Plant.
In battle, you will need to hit Metal Sonic 4 times to win, you can only hit him after dodging one of his attacks. You must keep running or else you will fall to your death.
1. The first attack will consist of Metal Sonic charging into you which can be easily avoided by jumping over him, afterwards simply catch up and jump on him to deal your first point of damage.
2. Now Metal Sonic will float in front of you and attempt to zap you will an electrical field. This is the hardest attack to avoid but can easily be recovered from regardless so dont worry. Once you avoid it, or get hit and recover, attack him again.
3. Metal Sonic will now snap off light posts and attempt to drop them on you. If you watch the group you can see the shadows of where the posts will fall, and can dodge them appropriately, hitting him when he stops.
4. Metal Sonic will now go berserk and charge at you from behind again but with far more power. You must keep running until a platform above you appears. Jump up onto this platform and wait for Metal Sonic to pass you, then drop down and finish him off.
Below is a video of mine to sum up what to do. Omochao also explains it.