This will be the hardest quarter of For the Alliance! for Alliance players on most servers. This is not because Thrall or Garrosh Hellscream himself is exceptionally hard to kill, but because Orgrimmar is typically highly populated with Horde players eager to defend him.
It is advisable to attack while a battle for Wintergrasp or Tol Barad is in progress, use the side entrance to Orgrimmar and head straight for Grommash Hold. This gives Horde players as little chance to interfere as possible.