Another way is to weld a threaded rod, or series of threaded rods to the dented area, heat the dented area, then the threaded rod/s goes through a hole/s drilled into a thick plate, a nut/s is attached to the threaded rod/s, and the nut/s tightened against the thick plate, pulling the dent/s out.
This way eliminates the need to drill a hole/s into the arm itself
It is then just a matter of cutting off the welded threaded rod/s & grinding the area/s smooth...Mark