Even though there are already multiple guides for this achievement, I personally still had a hard time casually getting close to 800 dashes using their methods. I tried using a metronome at various speeds with no result. Even when it felt like I was right on time, I would still drop out of my chain.

~What I did differently from different guides~

-Instead of using the mouse button to dash, I used a key on my keyboard. Personally I used ";" because I find that to be the most comfortable key to hit with my index finger. Your keybinds may vary by keyboard and its still perfectly valid to use your mouse buttons if that's what you find to work best.

-Instead of using a metronome to time my dashes, I opted to use a song I enjoy. This is the song i used. [goreshit.bandcamp.com] I believe this is the key detail that helped me achieve an 800 dash on my first try (first try using a new method).
[This song is 240 bpm and I would HIGHLY recommend using the same speed if you choose a song of your own.]

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

As for setup (image shown below), place your mouse cursor right in the center of the middle plate. Either the top group of three or the bottom should work fine, though I was on the top. Step a fair distance away from the plate to either the right or left and while facing up. Then, line your feet up with the middle of your cursor on an imaginary horizontal line. This will insure that your character does not bump against any walls during execution. From here, you can start dashing. Just keep in mind that 240bpm is too fast to start a dash, so you will have to build your dash off beat of the song to get things started. You should begin dashing back and forth with your mouse cursor in the center of your dash. Try your best to pay attention to the song and staying on time rather than watching the number go up. Nerves can kill an attempt so it's better to just focus on what actually matters. Once you hit 800 you're okay to drop out at any time. Good luck, this achievement is definitely an odd one.

https://steamuserimages-a.akamaihd.net/ugc/2506898018935202794/9414E07E67528BFE3F98C4BCF44874E90FA56150/?imw=256&&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false