You need the Rush Assassination Abilty at Level 3 and equip the pride of the lion sword you are given early on in your primary slot. Any assassination gear you have will help as you want to kill these enemies one by one in one shot.
Earn some adrenaline anywhere and Head to Rock Arch view point in Makedonia and quick save on top of the viewpoint just incase you mess up. Open your map and select the Ischagoras Military Camp near this viewpoint (to the right on the Map).
Run towards the entrance but not too close that they can detect you, wield your pride of the lion (when I did it without wielding with just the spear of leonidas it did not work however when equipping the pride of Lion like I’m about to attack it did) by pressing RT and aim a rush assassination towards one of the guards at the entrance (there should be 2), now all enemies will be out in the open in the camp so just keep spinning around until you see an enemie and spam the button Where you have put the rush assassination until you chain 4 times and the achievement will pop.
If you mess up just reload last save/quick save.
Update: You don’t need to kill the enemies just chain them