If I were you, I'd just keep it simple. WWsins are sweet, but you have to buy a special claw (Chaos) and in my opinion, WWsins work far better with gear other than Nat's. I've got a couple of assassins that use Natalya's, and the one that seems to be most capable ... she's simple. Tiger Strike and Dragon Claw are her martial arts skills (max them both and don't bother with the rest); max Claw Mastery and Venom; put at least a few points into Burst of Speed and Fade (as you'll have to, both being prerequisites for Venom), weapon block, and, if you feel up to it, direct a few points into Shadow Master, if you don't feel up to maintaining a mercenary but want a helper. No need to worry about traps on this build. (By the way, if you want, you can divert a few points from the two Martial Arts skills and put those points into Shadow Master ... sometimes having two of you rather than one seems more important than a few points' worth of damage. Personal preference entirely.)
If you do decide to go with whirlwind ... first, I have to suggest that you get some other items and put Nat's set on another assassin. See, it's my contention that the strongest backup skill to Whirlwind is Dragon Tail, particularly for PvM (which is pretty much all I do). The problem with Nat's set is that the boots aren't particularly powerful as kicking boots. About any elite boot, be it unique or just plain normal white boots (Myrmidon greaves?) kick harder than Nat's, and they Nat's doesn't give you any really good kicking mods. My WWsin uses Gore Riders (which, if I were on ladder, I'd have upped ... but I'm not so I haven't) that give, as you know, such great bonuses as: Open Wounds. And more! Also, the ideal second claw for a whirlwind assassin is Fury because it, too, comes with open wounds and other such great modifications.
But that's not what you asked. And you may decide that you want to go with a WWsin after all with your Nat's. (If you're on USwestNL, I could probably give you a good deal on a Chaos claw.)
One other thing you can do is just get eclectic with your skills. I'm sure Blizzard wants everyone to do this, the way they frown on cookie-cutter builds. But assassins are known to have several skills that could be called "one-point wonders" ... skills that you can just put one point into, and actually have a useful tool, rather than a skill that needs maxed before it does any damage at all (frozen orb? blessed hammer? meteor? etc...). Several skills come to mind here for the Assassin. Cloak of Shadows WILL help you in tight spots when there are things like archers running around. Blade Fury can do massive damage through Venom as well as whatever claw you happen to have equipped and as a bonus it steals life and mana. Mind Blast works wonders for instant crowd control.
So another idea, another thing you might try, is to just get a point into everything that sounds good, and see where you land. Inevitably you'll have some skills you like and some you don't, so you might consider rebuilding with these preferences in mind, maybe once you've gotten through nightmare.
Just in case you hadn't noticed, no mention of traps (aside from the blade thing) in this post. Really if you want to make a trapper, it's best to just build one dedicated to traps alone. You know. +3-to-traps claws and things of that sort. An inventory full of charms. 'Cause that's a character that will be useless for solo play, and Nat's set is, if nothing else, diverse in its intentions. No sense in locking yourself in to lightning damage -- especially since you'll be paralyzed by lightning immunes after you get through nightmare.
A final word of warning: you can't equip Nat's claw until you've hit level 79 which means you have a LONG time to get organized before you can get the full bonus of Nat's set (14/14 leech, +3 skills, +50 to all resistances, etcetra). This fact alone will make leveling her, especially solo, tedious and much more difficult than it has to be. But in the end it's your assassin.
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