Mixed sweepers traditionally enjoy +Atk, +SpAtk, or +Speed / -SpDef natures. Naughty, Rash, and Naive are probably what you should look out for.
There's your problem, lol. Use the Destiny Knot too and be sure to follow the IV chaining strategy I've laid out in the guide.
Neither parent has a 31 IV in Atk or Sp.Def (As explained in an earlier post). I took off the Destiny Knot so that I'd have a higher chance of getting the 31 IV in either stat.