UR R26R.5, VW Golf R
Oh yeah you get into that building via the adjacent building. If you look up before entering you'll see a path across which is a fallen steelwork or antennae that has collapsed. People say that mission was rock solid but I found it simple tbh. Kill everything on the way up ftw. Saves you doing it all on the way back down. You'll also need an item to help them jimmy open the lift door on the roof. Can't remember what it was but there's one in the building on one of the lower levels iirc. Some electrical component, weight of 10 I think.