Basically you need to kill the boss that looks like a "big dumb face".
I recommend the grenade launcher for this but I don't have all the weapons yet so other weapons may be better.
To reach this specific boss, you need to choose the path that has a one eyed flying 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, wait until it comes closer, and finish your ammo on it. Once all of its red projectiles disappear, exit the snake, max your ammo using the flowers, then finish it. Don't worry about it hurting you because if you kill it quick enough the projectiles will disappear.