Walkthrough:

Level 1:
Jump over the pit, walk to the goal

Level 2:
Walk to the right, and start jumping against the wall.
After you dematerialize and reappear, use your previous self as an elevator to reach the next level

Level 3:
Get hit by a bullet to knock yourself to the next platform
You can dash after getting hit if you need more distance

Level 4:
Fall down and shoot to the right, pushing your back against the wall
Rematerialize and fall down on top of yourself, getting hit by the bullet you shot last run

Level 5:
The easiest way is to do it without your echo.
Jump over the lower bullets and land on the platform. Then, try and time your jump between bullets to land on the platform below the lower turret. You will need to use your shooting dash to make this jump
This jump can be tricky, the timing is tight

Level 6:
Jump off the edge and shoot to the right
On the next run, ride your echo as you jump, and then jump off your echo into the bullet to propel you to the goal

Level 7:
Run up to the vertical tunnel and get hit by all 3 bullets, pushing yourself up. You wont reach the top
After you rematerialize, do the same thing. Your echo will be knocked up behind you, and at the apex of the tunnel you can use it as a platform to jump over the lip

Level 8:
This one is tough.
Fall down and shoot right roughly just above where the spikes start.
After you rematerialize, immediately dash to the right onto the platform, and then fall off the platform again immediately.
The timing should be just right to knock you through the opening.

Level 9:
Fall off the edge to the right and shoot to the right
After you rematerialize, fall onto your echo and dash to the right into the bullet
After the knockback is over, you should be close to the goal. Dash to the right again to nab it before falling

Level 10:
Jump and dash to the left
After you rematerialize, ride on your echo and jump while you are at the apex
After you rematerialize AGAIN, ride the echo that was riding the first echo. This will give you enough lift to get to the goal.

Level 11:
This level is slightly bugged, but still completable.
As you spawn, walk to the left and shoot to the left. Rematerialize instantly and walk to the left
Get hit by your bullet to propel you into the hole. At the bottom, dash to the right through the tunnel for extra speed
Jump and get hit by the top bullet at the apex of your jump. This should push you through the hole if you time it right

Level 12:
Not a real level, just a little playground as a reward for beating the game <3

The dialogue for level 11 and 12 are swapped unfortunately ;_;