Cuphead

I found this to be fairly easy on Botanic Panic, which is in Inkwell Isle I. It's the garden gate. While there are 3 bosses, none of them move and they all have extremely predictable patterns. I got this on my third try and I suck at this game.

As most of the other solutions state, Botanic Panic is likely the easiest level to get this on. To make it easier, just select the "Simple" difficulty, which cuts out the second boss altogether. I can confirm that this works as it is how I got the achievement. The Simple difficulty also seemed to make the two remaining bosses die faster, and seemed to make them slower overall, though I can't confirm either of those things; it's possible I've just simply gotten better at the game since the first time I fought them.

I've heard that you cannot technically beat the game on Simple, so maybe to be on the safe side, beat the battle on "Regular" first before attempting it this way. Since this game lets you replay levels and bosses, there's probably nothing to worry about, though.

Here is a quick way on how to get this achievement in the first Isle and I hope this helps out!

Level: Forest Follies
Weapons: Peashooter and Spread(can be purchased from Porkrind's Emporium for 4 coins)

For this achievement, you just need to make it through a single level without getting damaged and losing any HP. This can be accomplished on one of the first levels that you encounter after talking with the Old Kettle and crossing the bridge, Forest Follies. I used the weapon "spread" to ensure I had a wide range of protection throughout the level, although it does not have a very long range when shooting.

General Tips:

You essentially want to take out enemies as they come to you and take things slow, however, there are certain areas of the level that have multiple enemies coming at you and it would be a better idea to just run through those specific areas.

Holding down while running through the level can ensure that whatever you are aiming at will be taken out right away, especially when having "spread" equipped.

Make sure to utilize dashing with when in a tough situation.

Please reference the video for a visual of the level and what to expect.
Apologies for not having the level's title in the video

If you give me negative feedback please state why or PM me so that I can improve on my future solutions.

I unlocked this achievement 5 minutes after buying this game. It was the first achievement I unlocked. So it's easier than it looks or sounds... It only took me 1 level and a couple tries to a nail a perfect run. Even without the intention to unlock this achievement.

I did this on the boss level that you can access straight from the start. It's called 'Botanic Panic' wich has 3 fases: Potato, Onion, Carrot.
They're all stationary so it shouldn't be that hard to evade everything.
You don't need to have abilities unlocked, allthough the Invisible Dash makes it a little bit easier and will come in handy for those struggling to avoid those... uhm... Bullets?

The only upgrade I bought was the Invisible Dash ability. But it's perfectly doable without any upgrade too... Have a look at my flawless run

I made a quick video on how to easily get this achievement. Piece of cake.