first you will need to be on co-op and you must have sputnik skull

okay start off by u getting on to the roof tops and making your way to where the pelican comes and gets johnson.
do 'not' jump to the floor, instead make your way to the right side of the broken bridge

now get your partener to come to you if he has not come allready and use him as a step to
get on the building with the slanted roof and make you way to hotel zanzibar using the step technique(if your partener cant get up get him to kill himself).

once your there look for a small niche between the hotel and another building from here walk down until you see the edge of the map you can now grenade jump to the top of the hotel