Wani's Rough Guide to an Un-HP-washing Dark Knight

Unless by some miracle you get Berserk 30 before 140 (by earning or buying), I wouldn't put points until the 140s unless it guarantees you the ability to 2hko Skeles or Newties. Sure you can survive, but it's useless if all you're gonna do is kill easy mobs with horrible exp ratios or kill bosses for the rest of your life. I think stance is more useful before berserk, so I'd max that before berserk (staying on the topic this is unwashed). Leave stance at 20 as you described, then get berserk to 20 (or max it if you get the book by then), then do aura. Following that would be achilles, which I'd max instead of leave out at 20. In my opinion Dark Knights don't need MW20 until way later, if at all. I leave that to the other characters to use it for me. For me, the order would be in the later levels:

Max Achilles > Max Maple Warrior > Hex, if I even decide to get it.