All non mentioned achievement are story related, now lets start.

First letter is after a up down up down up, first bush on the roof
Second letter is after the first set of crab at the bottom left corner
After you activate the power and continue on with the game till a room with 4 turret. The letter is again on the bottom left where the turret cant hit you.
Super secret sala, sitiated in a vertical turret room in a one pixel gap that you do not see. It is right after the second turret
Finally for the last letter, it is situated when you go down a vertical line of turret on the third teleporter just keep going right

Heres a video of the speedrun Keep in mind that if you die, the crab cycle are gonna be different.

On this discusison : https://node2.projectplanet.cn/@steamcommunity.com/app/1428840/discussions/0/3004423845025478841/

the dev said that you have to 100% the game in one playthrough. Here it is :