This will save you a few minutes at best. Follow this precisely. Step one: Get Crossbow and Shield Step two: Type '2112' into the panel to your left Step three: Get the key from the drawer and unlock the curtain rope Step four: Raise curtains and turn the light onto the balcony Step five: When the gas comes, take the liquid from the panel and put it into the mouth of the mask Step six: Throw the grenade back at the operative while holding the shield to stop the arrow. Shoot the arrow operative with crossbow once arrow has been acquired. Step seven: Profit