From what I see, you have two pokemon weak to knock off, which is extremely common in NU, you do have Audino and Granbull though, so you do have your ways to counter it. If I were you, I would trade wish and heal bell for rest and sleep talk.
As for a 6th member, I suggest a pokemon that can set up entry hazards. Ferrothorn, Roselia, Pineco, and Uxie make pretty good options for this idea.
Also, if you seem to get setup on easily, a pokemon with roar or whirlwind would benefit Steelix does a great job with this, using roar and stealth rock, making sure to always survive one hit with it's sturdy ability, also a Quagsire would go great, with it's unaware ability.
All in all, I think STEELIX best fits your team. He is weak to fighting, which is sponged by Dusclops and Granbull. He is weak to water and fire, but Magmortar can handle fire, and Audino and Dusclops should be able to handle any water type in the tier. Seeing as your team is extremely bulky, I think you need to have heal bell on your team with Audino.
