Here's what Serebii.net says:
EVs: 84 HP / 252 Def / 12 Spd / 106 SAtk / 56 SDef
Now, I didn't follow that to the exact... I put 200 in Defense, and less in Special Attack (though it was in the 90's), and put those into HP. Even with less than 100 EVs in Special Attack, my Milotic has OHKOed her fair share of Salamences and Garchomps.
Both of them have a 4x weakness to Ice, and Milotic has a decent enough SpAtk as it is, so that combined with a 4x weakness makes it hard not to OHKOed. Besides, you'd want a ton of Defense so she can be switched into Salamence and Garchomp with ease (especially Dragon Dance Salamence), otherwise she'll crumble.
Either way, in my opinion, Milotic shouldn't be considered an attacker... she's a wall by all counts.
Oh, BTW, how do you count 429 Special Defense when her absolute maximum is 383? Granted perfect IV, max EVs, and beneficial nature? Marvel Scale only affects her Defense, not Special Defense.