Kill the guard. Find yourself a perfect spot. Lean your back
against the wall and kill them one by one. When you killed one,
drop your weapon and take his weapon. Do this every time until you
kill all of them (10 clones). Drag one of them to the place from
where they were coming out. Drag the body to the footprints. Enter
the door in front of you. Kill the professor. Watch the cut-scene
while 47 breaks his neck (that should have been included in the
game!).
(I first found the window where you can see them coming. That was
my spot to kill them all.)