MASTIFF ROOM - located right before you enter the cutscene involving the room full of brains, this part is quite easy to stealth, it requires simple timing, just make sure you have the RP grenade from the box before you enter. one mastiff will walk up the stairs, hide using a box in the corner where you enter, he will pass you, assassinate him. throw the red phosphorous grenade down into the center of the room and just before it explodes, time your jump assassination on the closest. make sure the other mastiff is facing you when you approach (obviously not too close of you will be detected.), he will turn around flustered and cautious in 90 degree turns, so when he does, get rid of him, this saves you from a very difficult fight.