If you're far enough into the game this becomes very easy to accomplish. Once you have taken over Vice King territory and completed all hitman missions, you have the penthouse crib and the platinum 21 round RPG in it. Since the penthouse is entered only via teleport, the cops can't get there, and you can have a breather any time you want.
So in short, get the rpg, blow some police cars up until you get up to a few stars of notority, and as they keep coming at you, slowly retreat back to the steps of the tower so that you hit 4-5 stars when youre there. When youre at the steps, you are too far from the main road for more FBI trucks to be generated at you, so youre pretty much in peace there. But they do send helicopters after you, so have fun blowing them up. Aim for the front or back, the middle opening is sometimes big enough for an RPG round to pass through.
IF you want a change of pace, just step off the stairs and 1-2 FBI trucks will come for target practice. Run out of ammo, no problem, just visit your weapon cache behind your back.