Thank you for the kind words. I'm surprised someone plays a game jam game 4 years later. The game is more of a puzzle game, than an action game. When I designed the stages I made sure they were doable without getting hit. I just replayed it to share the intended solutions:
Stage 1: go straight to the right into the enemy, easy to dodge the 4 projectiles that will have spawned
Stage 2: go to the middle of the first platform, wait till you only have to drop straight down on top of the enemy
Stage 3: jump as far right as possible, get the crystal and go underwater as fast as you can
Stage 4: get to the rock, jump on the platform where the chain is, can proceed without hitting the enemy
Stage 5: hurry to the bottom platform on the left, wait on the left side of it for the green danmaku to pass, go to the very left corner to jump up, delayed double jump to avoid the danmaku, then a timed jump up to get the enemy to jump to you
Stage 6: drop all the way down in the first part, once the enemy is at the bottom jump up to get them to come to you, then danmaku will stop spawning
Stage 7: should be impossible without getting hit once, to stay with the game jam theme "only once", iirc the route along the bottom is the most successful
You might still have to try a few times to get familiar with the timings. I wish you the best of luck should you decide to try again.