I'm a little confused about which champions I should be focusing on for endgame. 

I have Doompriest as a leader, Skullcrusher, Nazana and Athel, on Arena; but I'm still on Silver II. 

On the other hand, I'm thinking about growing up RK to 5* for farming. 

My boss clan team is awfull too, i tried it with Athel as a leader, Skullcrusher, Apothecary, Doompriest and Outlaw Monk.