Take a cab to the Helicopter location. Wait until you start to see people walking into the area and then pick a fight with one person at a time. Stay away from the security guards in orange vests and the police wont come unless one of the random people calls the police on their cell phone. Press B to push someone, then lock onto them with the left trigger. When they are about to throw a punch, tap the A button to dodge it. You will know you did this because Niko will sort of bend backwards. It doesn't count if you just move out of the way. As soon as you are done dodging the attack, press Y to perform the counter attack. You will know you are doing this correctly because it will be a series of punches and kicks that looks like a standard combo move. There is a hot dog vendor outside the helicopter entrance so if you are very low on health run out there real quick and grab some food. If you should die or get arrested, the clock doesn't reset so when you respawn outside of the hospital or police station just pick a fight with the closest person to continue your run. You can perform two combos on each person before you knock them out so just make sure that there are at least 5 people close to you when you begin. Some pedestrians will run away when you initiate a fight but most will not. Do this within four minutes to get your achievement. Good luck!