Twin Peaks Mountaineer

Strange... I thought it was common practice to grab the flag while buffed with berserking and grab the speed boost on the way out of the flag room while boasting your best /roar...

Best chance to achieve this (as Alliance) is to grab a Elixir of Water Walking and position yourself at the hut where the berserking power up spawns on the horde side. Drink your elixir, mount up, grab the berserker buff and charge off into the water taking the tunnel straight into the Horde flag room. Hopefully you'll face little or no resistance but have your CC's ready, snatch the flag, and race out the southeastern-most entrance to grab the speed buff. Just don't forget your macro for /roar and the achieve is yours, good luck!

IMPORTANT NOTE: Make sure the Horde flag is in their flag room.

And in case I need to state the obvious, if you have a water walking ability (looking at you DKs) you can skip the potion.

(I did this with my rogue who was able to sprint across the water since I was glyphed for it and this worked too, but I realized after the fact that you can grab the buff while mounted and speed across the water with a water walking potion.)

To clarify, since I couldn't tell from the comments here:

As a horde player, the speed powerup you'll need will be just outside the alliance base. If you're looking from the perspective of going up the ramp, it's on the left side of the building. If you're looking from the perspective of going down the tamp, it's on the right side. The side it's on, is the one that's farther away from the horde base.