Lets clear some things up,
FIRSTLY - Nor the Shaman or DK could've stopped the Boomkin doing that damage as he was in cloak of shadows. The only person who could've stopped it was the Hunter (scatter, pet cc or counter shot) - however he was just in the edge of the root/beam so I'm not 100% sure how many of those he could've cast.
Secondly, this will never work in high level Arena.
Thirdly, i'll explain how and why this happened and why it's 1. Not viable and 2. Not broken (any X class can fill the Boomkin spot in this comp and do the same):
Ok, so lets take a look at the procs he had (click for a bigger version):
+12% Damage (Nature's Vigil)
+288 Haste (Lifeblood) -Profession
+3670 Intellect (PVP Trinket 1) -Trinket
+15% Damage (Tricks of the Trade) -Rogue
+5179 Intellect (PVP Trinket 2) -Trinket
+15% Haste (Nature's Grace)
+20% Haste (Berserking) -Racial
+1650 Intellect (Jade Spirit) -Enchant
+Nature DMG (Stormlash Totem) -Shaman
+25% Damage (Incarnation)
+15% Damage (Celestial Alignment)
+3000 Mastery (Grace of Air) -Shaman Note: This is equal to +9.37%~ Damage because of Boomkin mastery
+5% Haste (Moonkin Aura)
Debuffs on Shaman
+5% Damage (Master Poisoner) -Rogue
+10% Damage (Prey on the Week) -Rogue
40% Haste (+288 Haste)
Now, notice how almost all of this was Enchants, Trinkets, the Rogue or the Shaman - and nothing outside normal CDs for the Boomer. He's running double DPS trinkets and used Cloak offensively for the opener. If the Hunter had done anything to interrupt one of the Boomkins casts and the Shaman trinketed the cheap and spirit walkers graced / ns'd / linked and survived the opener the game would've been an easy cleanup for the Hunter/DK.
Yes it's a cheese and yes it caught them by surprise but it isn't viable and is just a gimmick.