All you do is keep on jumping over the walls and eventually you will find a place where you can duck behind the pillars. Once you get over all of the walls run straight toward the pillars and crouch behind one of them. When the colossus catches up to you he will come to your level and look for you. Once he does that wait a couple of seconds and then run and jump and grab ontop his beard and climb up. The vital points are on his head and lower back. Make sure you get over the walls fast enough so he doesn't knock them down while you are on them.