Wushu is pretty darn simple (and also, IMO, ideally suited to a forum environment). The two basic ideas on which it hinges are details for dice and the principle of narrative truth.
Basically, it goes like this. On your turn, you narrate what your character is doing. Each detail you provide earns you one dice (d6). Thus "I shoot at him" gets you one. "I draw my pistol and shoot at him" gets you two. "I dive from the balcony, swing from the chandelier, catch the pistol Pete throws me, smirk at the hapless mook, then shoot him" gets you five. Scenes usually have a dice cap (often 6 dice per turn), so even if you narrate more than six details (which is fine!) you still only get six dice.
Dice are rolled against a target number equal to the stat you're using. So in the above example, if you were using Swashbuckler Extraordinaire 4, any dice that came up 4 or less would be a success.
Here's where the principle of narrative truth bit comes in. The actions you narrate automatically happen. You're not rolling the dice to determine if you succeed at them, you're rolling the dice to see how much further you advance the scene. If the Swashbuckler Extraordinaire rolls all failures, his action still goes ahead exactly as you described it, it just doesn't bring you any closer to ending the scene. The more successes you roll, the closer you are to resolving the scene in your favour. The more successes your nemesis rolls, the closer he is to resolving the scene in his favour!
There are only two caveats to this: 1) try to keep details appropriate to the mood and genre of the game, and 2) you can't describe killing a nemesis until you've earned that right (mooks you can describe killing -- there's plenty more where they came from! :)).
First off, I'm just going to quickly explain the combat rules, because they may help clarify a little. Each character (and each nemesis) has a stat called Chi. It represents your ability to affect the story. As long as you have Chi, you can use the principle of narrative truth to affect the game world. When you run out of Chi, you are at the mercy of your opponents. Players start with 3 Chi.
Now, when you come to roll your dice every turn, you need to split them between offensive (yang) and defensive (yin) dice. A defensive success negates one of your opponent's successes. An unopposed offensive success reduces your opponent's Chi by one. Nemeses have Chi and roll offensive/defensive just like you do. Whoever gets their opponent down to negative Chi first earns the right to a coup de grace -- your enemy is at your mercy (unable to affect the story), so you can describe whatever you want.
Note that this does not necessarily mean killing your enemy. These mechanics work for any sort of conflict -- in a chase, you might describe driving off to the sunset victoriously as your coup de grace. In a debate, you could describe the rapturous applause from your listeners as your arguments finally win them over. And hey, maybe in a fight your nemesis will just use his coup de grace to gloat over your battered body, before putting you in an Inescapble Death Trap. Whatever.
Mooks are a little different, since they exist mostly to make your characters look cool.
The first defence against this is trust. We're all adults here (at least I assume we are, and I'm going to treat everyone that way!), so we are relying on each other to work together to make it a fun game.
The second defence is slightly more rules-based. If anyone at the table (anyone, not just the GM) doesn't think an action you have narrated is appropriate, they can veto it, and ask you to take the action again (describing it differently). Preferably, a reason is provided with a veto. I have to say, though, that in the game of Wushu I played, and in all the ones I've read online, vetoes have never been used. We're all in this together, right?
I realise it's just an example, but that one is easy to get out of. First off, zombies in the bunker. Secondly, even if you narrate yourself into the bunker, if you lose the fight (ie, go into negative Chi) then you have lost the ability to affect the story. Realistically, your nemesis may be unable to get into the bunker, but a coup de grace doesn't force him to kill you. He could go off and kidnap your girlfriend while you're trapped in a bunker surrounded by a sea of zombies.
Wushu does sometimes have elements of one-upmanship going in it, and as long as you avoid logically indefeatable narration, that's a good thing. It's cool to throw a tricky situation at your opponent and see how he gets out of it.