IIRC the only pokemon with HAs unavailable are Fossils and StartersBulletproof is illegal, correct? I would say that Assault Vest would be this thing's best shot at surviving the moves that Bulletproof would have helped with. Fluffy is a really good ability anyway, I would say stick to what you're good at (Phys. tanking) and leave the Aura Spheres to anything else on your team.
I also believe that special Fighting coverage is going to be fairly rare this gen, with almost all physical attackers wanting that max move. Not only are fighting moves suboptimal on a dynamaxed special attacker, but a team is likely to have other Pokemon running Fighting coverage which will cause teams to run different coverage moves on their other teammates.