Some people use a hammer and a block of wood, some have used a strap wrench.
If you have never taken this upper tube off before, there is a good chance that the nut and stud that holds it to the alternator bracket is still there. It may seem impossible to get at, but I have found that you can access it from the driver side wheel well. There is just enough room between the frame and inside fender liner to insert a socket on a long extension to loosen, first the nut, and then loosen and remove the stud.