DISCLAIMER: I take half credit and throw the other half at Blizzard. Please note that I have no exceptionally high level shaman and that this guide was written a year ago. If I miss any important skills, please include them in your feedback.
I.DRUID
Drop down a snare totem, strip as many buffs as you can with Purge, and follow up with an Earth Shock to interrupt any spells. Then you have freedom to tank a bit; use your select choice of weapon enhancement and beat them down.
II.HUNTER
Much to my utmost shame, I've never actually killed one. I'd throw down and Earthbind Totem, use a Frost SHock, and then lay down various other totems. Usually I'm killed by any buds he might have. No comment.
III.MAGE
The best starter would be to drop an Earthbind or other snaring totem, with frost and fire resistance totems after that. If o when you can, Shock them to interrupt spell casting. You may want to bring some potions. Mages can be a real bother, as they typically root you. Heal if you have to, although it's much faster chugging down a potion. Once free from the root, close the remaining gap and snare or interrupt, or you'll be dead.
IV.PALADIN
Again, use a sanre totem and kite them as long as possible while tanking and occasionally shocking. Try to force them to use their shield, then cast Purge, Kite them with snares until you feel you can finish the battle by beating them down.
V..PRIEST
Purge the shield whenever they use it and drop a snare totem. Then ,you get an opportunity to tank a little. Use Shocks often to interrupt spells.
VI.ROGUE
If they manage to jump you, this is going to be a difficult battle. Drop a totem, chug a potion if you have to, shift into ghost wolf and kite or flee. If you can jump them, Sanre totem, kite with spells and more snares, and close in for the melee kill.
VII.SHAMAN
Now, if you're dueling a shaman, this might be a little tricky. Drop as many totems as possible, and use Shocks often. Try kiting, or close in for melee when they're at about 40% health and chug a potion. You might want fire and frost resistance if they choose to enhance their weapons.
VIII.WARLOCK
This is a battle that can be surprisingly easy, with the exception of fear. Snare them, ignore the demon, and take them out with melee as soon as possible. They're at the lowest part of the totem pole(no pun intended) when it comes to melee. Take into account, however, that it will be much more difficult if they manage to fear you. For this purpose, use shocks.
IX.WARRIOR
Warriors can be named Shaman's Bane. The only thing I recommend is to snare, kite them, and only jump into melee if you're feeling stupid, suicidel, crazy, or any combination of the threem. Flee and come back with comrades.
Again, this guide is expressively based off of my experience and opinion. Please include suggestions in your feedback.