Ok, how about this for a solution (one that I will probably be putting to use for my own shotgun:
Load your tube with buck shot (in my case, 20 ga #4 buckshot). However, load the last shell/chambered shell in birdshot of your preference. That way, if the first round disables him, you have no worries, and if you need more than one shot, then you have serious stuff backing it up.
Internet assassins, commence!