After a series of guides featuring 'pure' classes, especially the past weeks' pure DPS rogue, it should be noted that shamans are an exciting hybrid class from a purely PvP perspective even though they only fill two of the three archetypal roles. This is because the three specs fight considerably differently. Elemental shamans are casters in mail armor, and always prefer to fight at long range. Even though they have access to instant cast shocks, their repertoire will consist mostly of Lightning Bolts, Chain Lightnings, and Lava Burst, all of which have cast times and directional facing. Get in their faces, get out of line of sight, and generally try to interrupt their rhythm. An elemental shaman will always apply Flame Shock before casting Lava Burst, as it guarantees a crit. If you can remove magical effects, such as with Cleanse or Dispel Magic, be very attentive to Flame Shock and remove it as soon as it's applied. It will be easy to identify an elemental shaman because she'll be carrying a one-hander and shield with Water Shield up.
Elemental shaman are excellent kiters and have some cool tools that allow them to create some distance between themselves and their opponents. Earthbind Totem is a powerful spell that roots opponents for 5 seconds, thanks to Storm, Earth, and Fire. With Earth's Grasp, the totem can be cast every 10 seconds, allowing root about 50% of the time, subject to diminishing returns. The drawback? Elemental shamans have very little by way of snare removal themselves, so they can be slowed and rooted as normal. Physical and magical crowd control works best, as elemental shamans have no way of removing this aside from the PvP trinket.
They also have an excellent peeling spell Thunderstorm and you should always be wary of fighting them in elevated areas such as the lumber mill in Arathi Basin or on the Dun Baldar bridge. Aside from this spell, which has a 45-second cooldown, the general idea is to get up close to an elemental shaman and disrupt her spellcasting. Even though they wear mail armor, they have few mitigation abilities and can succumb rather quickly to various forms of damage.
Aside from shocks and Thunderstorm, elemental shamans won't have anything instant to worry about, so just keep on disrupting their spells. Their heals all require cast times and their damaging abilities require them to face you, so managing to position behind them will ruin their momentum. Note that stunning, fearing, or silencing them will only proc Astral Shift, so unless you can unleash major damage, it sometimes isn't worth the bother.
Enhancement shaman are the easiest type of shaman to spot: they're dual-wielding weapons. The odd enhancement shaman will be carrying a 2-hander, but most will have two weapons. As opposed to elemental shamans, however, who have basically one playing style and a rather narrow choice of PvP talents, enhancement play styles differ -- some go for faster shock and Wind Shear cooldowns to harass casters even more, others dip into restoration for faster heals. Overall, the strategy against an enhancement shaman is opposite that of an elemental shaman -- kite her.
This will be a tall order because enhancement has Earthen Power, which turns Earthbind Totems into snare removers. Here's a tip: it only removes slowing effects, not roots. This means that abilities such as Entangling Roots and freeze effects won't be removed. Don't bother slowing an enhancement shaman. Root him if you're able. Barring that, make sure to quickly target the Earthbind Totem and quickly reapply a slowing effect, although Earth's Grasp will allow the shaman to put down another one relatively quickly.
An enhancement shaman can become truly dangerous when her wolves are out with Feral Spirit. My advice? Stay clear of them. Unless you have some means of crowd controlling them, such as with a mass fear or even a Hibernate on one, enhancement shamans with Spirit Wolves out can deal a whole lot of damage while being harder to kill at the same time. The wolves have fairly high health and it would be a waste of time to focus your attention on them. Unless you're confident about your crowd control and/or ability to take some damage, it's easier to avoid them at this point. If they manage to stun you and keep you in place, and blow cooldowns like Bloodlust or Heroism, they can take down pretty much most opponents with a flurry of magical and physical damage.
There is no strict counter to an enhancement shaman with Feral Spirit active, other than perhaps to crowd control the shaman herself and attempt to take down the Spirit Wolves before she breaks free. Enhancement shaman are fairly susceptible to crowd control and have no natural means to remove it other than with a trinket or the right totem (so make sure to kill it). Keeping the shaman out of play is your best bet, and if you're unable to do that, it might be wiser to run away for a while.
Smart enhancement shamans will keep stacks of Maelstrom Weapon ready for instant self-heals instead of going overzealous with damage (although they can certainly do that, and it will hurt). If you have offensive dispels, use them on the shaman to prevent the stack from reaching 5. Most shamans aren't in the habit of consuming the buff if it hasn't reached 5 stacks because casting, however fast, resets their swing timers. Most play it safe and use the stacks for a heal at around 50% health or lower. Disarm effects work well against enhancement shamans, although they still have access to damaging spells and a few will have a weapon chain on their off-hand weapon to counter such an occurrence.
Restoration shamans are identical to elemental shamans in that they will be sporting a one-hander and a shield. A tell-tale sign would be if they have Earth Shield around them, although most healers will throw this at another player instead of themselves. When under fire, they'll apply the Earth Shield on themselves, and keeping it off will be key to beating them. It's not a cheap spell, and it will cost them heavy mana to keep on re-applying it. If you don't have offensive dispels, you'll have your work cut out for you because Earth Shield will consume a large chunk of your damage.
Because of their support role, restoration shamans are masters of Purge. They learn how to use it more effectively than either of the other two specs because restoration shamans look to strip opponents bare for their teammates. Restoration shamans with a bit of Arena experience are well aware of enemy buffs and watch for openings. By the same token, restoration shamans often learn to /focus with Wind Shears by way of supporting their teammates, so always be wary of casting anything against them even if they're healers. As with any healer, they are fairly difficult to kill, especially with abilities such as Nature's Swiftness to give them instant recovery.
Whatever the spec, shamans have a common weakness in that they are fairly easy to crowd control and cannot remove magical effects on themselves (although restoration can remove Curses). Aside from restoration, shamans generally have casting times on their heals, so they can be interrupted, locked out of the Nature school, and slowed down or prevented from healing themselves. Sometimes, these encounters come down to a mana battle and running the shaman out of mana will usually result in a win. They have various means of recovering mana, but the only one that can truly be countered is the restoration shaman's Mana Tide Totem. Kill it. No, seriously, don't even let it tick more than once or twice. Thunderstorm and Improved Stormstrike can't be stopped (and return just a little mana), so don't bother.
For all the danger that shaman burst poses -- and they can kill just about anything if they pour it all on -- again remember that they are completely vulnerable to crowd control. Make sure that you eliminate any of their defensive totems and control the match with any crowd control ability you have, however limited. Defensive totems are preventive and they can't cast them when they're already cc'd. Fears, stuns, charm effects, sleep... you name it. The only thing shamans can handle with relative ease are snares.
Anyhow, that's it for shamans, I think. I might have I missed something, but you guys will set me straight in the comments, anyway, so I'll sit back and let you toss your own strategies against shamans. That's always the fun part. Start hitting those battles now, because it's going to get a lot more fun with Patch 3.3.3.