The stipulation here is that it is an Offensive Shaman used soley for the purpose of killing Druid FC's. In this case I would take Orc as it has the best damage racial, but troll is a viable option as they will do more damage to beast and can speed up attacks when they finally get on the efc. Tauren for interupt is ok, but the stun is imo not as useful as all the damage increase every time you get on the efc with the others...really any of them have benefits.
If you have 5 agms finish the orc bud, I already roll 2 tauren shaman in addition to a bazillioin hunters in premades haha. My 2 d shaman roll tauren for warstomp and engineering for bombs, we interupt everything we can to slow the offense roll.