I killed all the badguys in the office of the levelboss and after hé said that he opened a door for me, I can't find this door.
And I sometimes I get shot by 2 snipers I guess, but can't find them.
Are you still in the office? The door is pretty obvious in the corner. Or are you out in the corridor? There are several snipers across on the other side of the garden. If you've still got a sniper rifle, they should be pretty easy to pick off. Otherwise I think there's one there. Or use a Scar.
I died twice on this level, but that was just trying to find where the other snipers were. If you can't find the door in the same room as the old man you are blind.