Cuphead

I found this way easier to do in the casino rather than the first boss. I recommend doing this on the racing horse boss. You'll have to beat one round of bosses to get to him because he is on spot 6 but that also means you can snag an extra heart before facing him from landing on the heart spot in the first group of 3 bosses. He is a super straight forward boss with way less health. He only has two attacks. Dodge the present that he shoots out of his chest. Then dodge the horse shoes hat come out when it explodes. Eventually a blue hooded dude will appear and you will have to dodge him too. He will charge up at you when he rides one of the horses moving below directly underneath you. Also if you're having trouble getting it with this boss another option is the toy monkey, he is on the ninth spot. Which ever you are more comfortable with both these bosses are way easier and shorter than doing any of the other full stage bosses.

Here is a video by Zasta 360GameTV fighting the boss

This achievement is for a whopping 0 gamerscore! Let's start:

I would recommend doing this on "Threatenin Zeppelin" on simple difficulty.

This fight consists of five parts:
Hilda I
Bull
Hilda II
Twins
Hilda III

As soon as the boss fight begins switch over to shrink mode. You will have to stay in this mode while doing damage for the entirety of the level. Don't worry if you switch back to normal cuphead. As long as your not doing damage with normal bullets then you won't have to restart. This boss battle should take between five to six minutes.

Credit goes to Foamy036 for the tip - If you are struggling to stay alive for the duration of the boss battle consider picking up the twin hearts charm. It will lower your damage a little bit but it will give you two extra hearts!

Hilda I - Hilda will attack with "HA" in the shape of bullets every few seconds. Watch out for these and bullets from the planes. You will need to get somewhat close with the shrink plane to do damage. I would try to find the sweet spot. The sweet spot is the maximum range where you are still doing damage to the boss. Once Hilda has taken enough damage she will inflate herself and fire in a straight line. Avoid her attack and stay on the far left side of the screen.

Bull- Every ten seconds the bull will stop and then charge at you in a straight line. Avoid these attacks and try to do damage whenever possible. You'll have to watch out for the green planes. They will shoot three bullets in your general direction. This can make it tough sometimes because you'll try to avoid the bullets and end up getting hit by the bull. If you made it out of this phase with at least two HP then your in a good position.

Hilda II - Hilda will have another attack in her arsenal. Every ten/fifteen seconds she will throw a tornado at you. When she does stay at the left side of the screen near the middle and find an opening on the bottom or the top of the screen to avoid it. Otherwise, the fight is a lot like her first phase.

Twins - The twins will dance around and summon an orange ball. The ball will shoot out a laser and move in a 360 degree motion. Avoiding the laser is fairly easy but it's tough to predict it's starting point. However, If you stay to the right of the ball when it's firing off a laser it won't hit you. This phase is pretty easy for you once you get the hang of it.

Hilda III - Your almost there. Hilda has become more aggressive with her attacks and the planes will come out faster. Be patient when dealing damage to Hilda. You'll have the urge to deal a lot of damage to her early especially if you have a couple HP left but you can lose your health really fast. Keep doing damage and she will eventually fall.

Now you're 0gs richer!

Credit goes to Maka91 for the video

This solution will only be useful if you have the DLC: The Delicious Last Course (Since I understand that the genie can only be summoned if you have this DLC, if I am wrong I will change it as soon as I read your comments)

If no other solution has worked for you, this can relieve a lot of pain.

Go to Inkwell Hell and Summon the Genie, to summon him you must spin with Cuphead, once there Play the "King Dice" level with Miss Chalice. As you can see you have more lives! Therefore, this will increase the chances of achieving this difficult achievement, choose the phases that suit you, but you must necessarily make the boss of box No. 6, this is a not very complex boss and is the best option when it comes to wanting achieve this achievement, once there you will have a greater chance of losing lives and thus achieve the achievement, more easily. Anyway, I leave you a video guide to the achievement (And as you can see, I don't roll dice in the casino and that's why I fight with more bosses hahaha, the fight with the boss itself starts at 48 seconds)

This is my video, The video is in Spanish but it is just text: Fight Start at 48 seconds


Note: You can also play as Cuphead or Mugman and equip Double Heart + Genie Summon 🏻 If you liked the solution, I would appreciate it if you left a thumbs up 😁