Building stamina takes perseverance which, in a way, is like saying it takes stamina to build up stamina. The Stamina Builder workout is exactly that. There are specific things to watch for here: Intensity (it shouldn't change throughout each 3 min period). Form (try and maintain as close to perfect form as you can - on wall sit, for example, your legs have to be at 90 degrees), Pace (go through each set at an even pace that maintains intensity. Do not give your all in one exercise and then flake out in the next). EC is your goal here. Really.
Extra Credit: Last full 3 minutes each time.