In Level 4 there is a checkpoint where you first have to jump over a small crack and then you can see parts of a ruin. You walk up straight to the door and just run inside. Shoot at the first guy, run towards him and hit him. Focus the next guy and do the same procedure again – there should be precicely 5 guys but if I have miscounted … just let the last one kill you. Only possible in Easy game mode but I succeeded at first try.