Haven't multiboxed with my chars yet.
However, sometimes the alliance partymembers will listen, and then we'll win.
Following, not my idea, read it somewhere else:
Make sure the group sticks together all the time.
Make a run to the horde flag (all together). one tank or someone with armor takes the flag.
Still stay together, no speed stuff or whatever or the effect will be lost
Horde has captured your flag.
Make a run, still all together, for the horde carrier. Kill him, return flag to base.
Run to base, if it is captured, repeat, until you are at base at can finish it.
Repeat this tactic 2 times.
I've won quite a few WSG like this, in less then 6 minutes the game can be over.

Maybe you can do it with your 5 shamans...not sure.

Greets,