Use the fire escape to get better angle of Donnu's office, when he enters jump in and take him out quietly.
From Create Your Own Story
Mario and you climb the fire escape to get a better view of Donnu's office. Luckily for you the fire escape begins at his office window. A few minutes later Donnu enters his office and sits down behind his large desk. Choose a silent weapon to finish him with. DO NOT USE A FIREARM!
