This can be easily accomplished in the first "true" level of the game. When the bungalow is assaulted and the lifeguard is killed, go to the back door. Pick up the gun on the table
and put it away
. Now your fists are out. Open the back door and punch out the guard on the deck
. Grab the key from the table and proceed out the front door. Do the same to the guard here. Proceed down the pier where the helicopter was and do the same to all the guards here. If you don't have the achievement by this point, proceed back up the pier and around the winding path, taking the time to only engage the guards within striking distance. Ignore the cliffside guards and by the time you follow the only possible path around to where the helicopter lands yet again if you punch out one or two guards here you should unlock the achievement.
It's worth noting that this is very easy on "Sleepwalk" difficulty as you can just charge the guards within striking distance with few problems and heal yourself with plentiful medkits in the area if need be.