Against a physically defensive Skarmory, it's 70.66%-83.23% (a 2HKO). Against specially defensive Skarm, it's 47.9%-56.29% (2-3HKO). Unless you want to run a -Speed nature, you're gonna have to lower one of its defensive stats.For the first question, basically any Steel-type that can't be taken out with Superpower (ex. Skarmory). Second, I can change his nature to something else (not sure, possibly neutral because I need my defenses even if they are awful).
So, slap on a Life Orb and replace Night Slash with Heat Wave?
And yes, just convert to HP EVs into Special Attack EVs.