Once you take over the two Rackets in the downtown area you'll have your choice of two objectives. The direct option takes you straight to the hotel to confront Derazio, however, I recommend the second option.
Head to the other objective marker, kill the four enemies there, and steal the armored car. Drive it into the underground garage at the hotel and park it. Take the stairs up as high as you can go without engaging enemies to make this initial part of the mission easier. Eventually you'll have to enter combat as you approach Derazios penthouse.
After taking him out - which really isn't that hard as long as you've got decent weapons - head back down engaging enemies as you go. Eventually you'll return to the stairwell you came up (it should be marked "garage") that you can use to return all the way to the garage level without much resistance. If you don't take these stairs you'll have to fight through a couple floors of enemies. Just as you avoided those floors by using the armored car to get into the garage, you can head straight back to the garage and avoid these floors on the way down.
Clear the garage of enemies and use the armored car to escape the garage and the vicinity of the hotel to complete the mission.