Basically you need to kill the boss that looks like a "big dumb squid".
To reach this specific boss, you need to choose the path that has a fish monster next to it at the beginning of the run.
Once you reach the boss, don't exit the snake (the tube thing that teleports you between levels) to stay invincible. Press once
to stick your head out to be able to shoot at the boss. While inside the snake, shoot the tentacles that cover its eyes and then shoot the eyes (only when they are open). When your ammo is about to run out, exit the snake and get some ammo from the flowers while avoidig the little monsters, then finish the boss.