How the fuck do u beat the dude in the dual i did it 4 times my dude wont fire at all and he jsut kills me each time really pissing me off.

wait a bit after it goes into slow motion before drawing, it'll make your shots more accurate and make the bar go up quicker. Just like any other duel.

Same issue, He fills up his bar in one go and blasts me all to hell every single time.

just click  really fast. no big deal 

Each shot placed fills the bar.
 
However, placing shots on the gun or the head lands a white crosshair mark, and fills it to max. Usually if you place it right, you only place 1 shot and it ends immediately. (Note: some duels have to be fatal, so disarms won't be enough, you'll have to shoot'em dead.)

Two in the sternum, four in the face.  One more to the head after the duel for good measure.

Yeah, i tried disarming him but after a few deaths i just killed him, that seemed to work

It's not difficult.
 
Your reticule turns from red to white. Red is inaccurate, white is dead on. Place your markers accordingly. Each shot that hits fills the bar, whoever fills the bar more, wins.
 
Just put a white marker on his forehead, and you can win in one shot, every time.

The whole 'has to be fatal' thing held me up with Muller. I tried to disarm with Eva's mission too in case there was more story for her pimp to give. In the end though a bullet in the groin and the rest in the head did it.

I tried to disarm Muller in that duel, and bit the dust. Haha. 
 
On this one, however, this guy got six bullets. One in each kneecap, one in each arm, one in the chest, and the last in the face. He deserved to die for what he did.