In the settings menu, you can go to the accessibility tab and customize the game and the enemy bullets speed. While I'd personally wouldn't change the game speed as it changes your own movement speed too, slowing down the bullets is helpful. Less than 40% made things harder though, as bullets stayed too long on screen. Find settings that suits you, and it will makes the achievement way easier.
There is a setting that will make you invulnerable (In HP regen assist), but the game states that it disables some achievements so I wouldn't try that.
Doing this on the first level is the easiest solution. You don't have to visit every room, only the mini-boss and the boss rooms are mandatory.
I found the boss to be the trickiest part. Even if he isn't all that hard, here are some tricks :
-Be careful about the spikes.
-When he shoots bullets down, use
to slam it back. Slow bullets makes it easy.
-When he goes to one of the side, if he is at ground level, double jump (Slash with
if you timed it wrong) over his hand. Else you should be fine.
-When he goes to the center of the screen, a "!" will appear and he will fire bullets around him if you don't stop it. Jump on him and use
to stun him, and use the opportunity to do good damage. I wouldn't even really use the sword when he wasn't stunned.
-Use your gun every time you have ammo.
-At 40% HP, he will gain a new attack where he slams the ground, shooting bullets in all directions on impact. If you see this, stay on one of the side. I found it easier to jump between bullets than deflect them.
-At this point, all of his attacks will shoot bullets on impact, and bigger ones will appear when he goes from side to side. You can deflect them, so it doesn't change that much.
-If you have unlocked the Fury (
), use it when he drops in this second phase. If you do that, a full clip of you gun + one stun should kill him. With a little bit of luck, he shouldn't even be able to attack.