You'd probably want 0 Attack IVs on Magnezone, Shuckle, and Thundurus. It'd reduce damage from Foul Play and damage you'd take if you hit yourself in confusion. Also possibly 0 Speed IVs on Magnezone, and maybe Quiet nature as well, to take advantage of Analytic. You don't need 6 EVs on Registeel, Magnezone, Shuckle, and Dusknoir. It has the same effect as 4 EVs. I personally think that Registeel should have some sort of recovery, like Leftovers as an item instead of Silk Scarf. Though I know your set's also a nuke, I don't see what Amnesia would be for. It boosts your Special Defense quite nicely, sure, but you can be whittled down as you can't recover any HP with that set. Also, Registeel is helpless against Ghost-types, as Ghost-types are immune to both Explosion and Brick Break. Sure, you'd probably switch out to Yveltal in that instance, but it still seems like a bit of a problem to me if this is some sort of offensive tank. Usually 252 HP is better than 252 in each Defense stat, too, except for Pokemon that have really high base HP. I'd say Registeel would benefit more from 252 HP. Shuckle as well. Then you could use the rest of Registeel's EVs for either Defense or Attack investment, though Registeel doesn't seem like much of an attacker to me. Payback and Shadow Punch on Dusknoir? They cover about the same amount of Pokemon. Thunder Punch provides more coverage, and nothing is resistant to a set of Ghost/Electric/Ice/Fire, while eleven Pokemon are resistant to a set of Ghost/Dark/Ice/Fire. But if you do want to run Payback, 0 Speed IVs could come in handy, because the slower you are, the higher chance Payback could deal more damage. Also, Thundurus is missing 4 EVs, I just want to say.
Though I'm not a really skilled competitive player, so you don't need to take my advice. Just some of my thoughts on it.
