This is achieved when you win a leg of 501 darts in 9 darts which is the fewest you can do. This can be done in exhibition, career or tournament but not in any party game.
There are many ways to hit a 9-dart finish. You can hit 7 treble 20's followed by treble 19 and finish on double 12, you can hit 7 treble 20's followed by treble 15 and double 18, hit treble 20, treble 19 and bullseye 3 times, and this just names a few ways to do it.
The easiest way I found to do it was to hit 7 treble 20's in a row by aiming at the very top of the board as far as it'll go, then gently flick the right stick to give it as little power as possible and more than often it should land on the treble 20. After the 7th treble 20 when your left on 81, move your cursor to the bullseye on the center of the board and move it slightly to the right beside the outer bull, this should put the cursor at the narrow part of single 6, again flick the right stick with little power and it should go into treble 17. This will leave double 30 and here you again move your cursor to the right and aim for the large part of single 6 and again flick the right stick to give it little power and it should drop into the double for the 9-dart finish.
This sadly won't work every single time due to the cursor moving by itself on finishes to simulate pressure plus the dart will sometimes be deflected off the target by a dart already on the board, but I did 9-darters this way more times than doing it the traditional way of treble 20, treble 19, double 12 and found double 15 much more easier to hit than double 12.
Also I must warn you that the achievement is buggy and may not unlock for you first time around. I remember I had to do about 5 9-darters for it to unlock and having recently gone back to this game for the Grand Slam achievement I tried this on another account and I needed 3 9-darters for it to unlock. So if it doesn't unlock first time don't panic, just keep hitting 9-darters and it should eventually unlock.