Hyper Light Drifter

First things first, you need to buy the chain dash ability from the dash shop in the town area. This is the ability where the video tutorial tells you to press + . Then, you need to go to the large room to the right of the dash shop, where you can practice your new dash ability to your heart's content. When you're in this room, there is a white counter at the bottom of the screen that keeps track of your most recent chain dash combo, and a pink counter that keeps track of your personal record. There are four things that you can unlock in this room by getting a high enough dash combo:

- The "Chain Dash Champion" achievement unlocks at 25 dashes in a row
- A gold piece unlocks at 100 dashes in a row
- "The Dash Eternal" achievement and a new color outfit unlocks at 800 dashes in a row

Now, getting 800 dashes in a row is no easy task. It took me about 3-4 hours to finally do it. In order to do a dash combo correctly, you have to time your button presses. The timing is pretty lenient, but if you mash the button too fast, you will stop dashing, and if you take too long to press the button, you will stop dashing. Not only that, you have to navigate your character around the room so that you don't run into the walls, because if you do run into the walls even a little bit, you crash and stop dashing. When I finally completed the achievement, I was hitting the button about 4 times a second, and I traveled around the room counterclockwise, doing my best to stay in the middle of the track. It took me about 3 minutes and 8 seconds to get to 800 dashes.

I found a video on youtube that shows off a similar strategy to mine (It's on the PS4, but the same methods still apply):

UPDATE:
Thanks to Awoo's comment, I found that I could more consistently complete this challenge if I muted all game audio, turned on a metronome to 240 bpm, and pressed the button whenever I heard a beep. I was also much more consistent when I dashed back and forth 2-4 times at the bottom of the room. Just choose whichever number you are most comfortable with, then stay consistent. I personally found it was easiest to dash back and forth 3 times in each direction. The following video shows my strategy off:

Also (thank you to Red Pill Chosen for mentioning this), if you still are having trouble with the achievement, you can find and equip any part of the green/blue outfit, and it will cause the timing for each dash to be more lenient. Information for finding this outfit and any other outfit in the game can be found here:
http://hyperlightdrifter.wikia.com/wiki/Outfits

Finally, it's best to choose whichever strategy that you like best, and then be consistent with it. As long as you keep trying, you will eventually be able to get this achievement. Try not to worry about how far you are; just zone out, and focus solely on your timing. You will know when you reach 800!

Good Luck!

This challenge was gut wrenching. My top score is 831 using a Xbox One controller. I turned the in game sound and music off to avoid distractions and traveled in a counter clockwise direction around the room. At about 700 I thought my heart was going to fall out. Haha. I have a 5 minute video I recorded of it in this link:

(Yes this is my actual run at it)
One thing that may help is there is a cape/sword/bot you can wear that makes the window for pressing the button a little more lenient. Of note is that you only need to equip one of the three to get the effect. (Meaning you can equip three different items and receive three different status effects. Green/Blue Cape (Crystal traps-dash challenge)= increases milliseconds for the input window of chain dashes. I didn't find this out myself, however, and would like to thank the steam community. Namely CYBERfailzor, for his post on the topic.