This one is a bit tricky and could take you quite a few tries to achieve as you are unable to set of any alarms off. So go around the level sneaking and taking opt shots at enemies, there are several alarm systems, about 4 before the bomb and there is one AI next to each one somewhere. Make sure to sneak and take them out first. When coming across the hostage leave him there for the time being as when you untie him he runs to the window and sets all the alarms off. Once your in the building with the bomb you'll locate Kwennoir downstairs next to it. From the stairs you could lob in a few grenades or use the grenade launcher on your gun to fire into the corner room where the bomb is a well placed shot should kill him.