I see you're an FFXI person ... so let me dare to give you an FFXI analogy.
Rogues are the ultimate damage dealer but depend on strategic physical positioning to be most effective. They also have no magic defense and no healing.
From my Ballista experience in FFXI, in the odd times when I'd get the change to duel someone, I could always take the DRKs and THFs. I would just enfeeble the hell outta them, buff myself to the tee, and hack away. If their friends didn't come join in, I'd kill them in about 1 minute. They'd just keep missing me, hitting my Stoneskin, and when they did get any damage through, Phalanx was right there to absorb 1/3 of it. I could heal when necessary, and unless they happened to be DRK/WHM (a good Ballista combination, strange as it might sound) they'd quickly die. Oh - and good luck Aspiring a mage with much higher magical DEF than your magical attack.
Same idea here - Shamans have healing, buffing and enhancing types of magic. While you could probably take him out if he couldn't heal himself - he can and will, and all your hard work won't help unless you can kill him fast enough.
I'm a Hunter, but I've never beaten a Shaman. I've come very close. The best strategy I've found was to just stand at range (which Shaman's aren't opposed to) and just fire as fast and hard as I can, and send my pet to attack him and destroy his totems.
Now that you brought all this up, I'm going to go duel some more Shamans and see if I can come back with any new insight.