In the game statistics its listed as SINGLE TRIGGER CHAIN!. What you have to do it kill 99 people without lifting your finger off the trigger on your weapon. Almost impossible to do this with hand held weapons. Best way to do this is while being in a tank.
To get a tank, you will need to reach maximum threat level so that the Army will start chasing you. Fastest way to raise your threat level is to steal a vehicle and start running over pedestrians. The higher your threat level is, the more damage you take on, so keep an eye on the vehicles armor. When the vehicles armor is low, quickly get out and steal another vehicle and continue your rampage until the tank appears. When the tank appears, simply run over all surrounding soldiers, jump out of your vehicle and get inside the tank.
Once in the tank, hold to fire missiles and DO NOT LET GO! The tank has a great amount of armor. If you stop firing at any point, your chain multiplier will reset. Just keep shooting till the achievement unlocks.