In rampage one you have to kill at least 25 Rednecks within 2 minutes only using an assault rifle and grenades.
How to Regenerate Health
The easiest way of doing this is to utilize cover wisely, however don’t use the cover mechanic to do so. Stay behind things such as cars. The mini map will tell you where the rednecks are and where cars are coming from. Aim for the head for a quick kill.