I would suggest to let this achievement be your last one because you will gain a lot of shots by playing through the game normally but incase you want to speedrun this, here's how;
Fastest way to do this is going to custom settings and changing "max shots per round" to infinite , "max time per round" to infinite , "max time penalty" to 0 and "retry shot mode" to "from start".
Now you are able to spam the shoot button and "R" button to reset extremely fast (remember to shoot first and then press "R" button).