Personally i would look to get an attack down champ in there as a priority as it will greatly improve your survivability on the clan boss, unless one of those champs has it and i dont realise it?

Getting your arena nuker out of clan boss is a good idea by the way

Yes.  Put  in  Gravechill  for  Kael.  Make  sure  everyone  has  Warmaster  (except  Apothecary  should  have  Giant  Slayer).  Frozen  Banshee  would  be  your  lead.  Stack  Defense,  Accuracy,  and  Speed  on  FB  and  GC.  Stack  Speed,  Defense,  and  HP  on  Apothecary  and  Gorgorab  (add  Resistance  for  Arena  if  you  want  to).  Turvold  can  be  built  full  nuke  with  Attack,  Critical  Rate,  Critical  Damage,  Speed,  and  Accuracy.  Turvold,  GC,  and  FB  should  be  in  Lifesteal.

Decrease  Attack,  if  you  have  someone  with  it,  should  replace  Gorgorab  (and  you  can  use  a  more  Arena  optimised  build  on  him).  Bogwalker  is  a  common  one  often  overlooked.