Cuphead Bosses Ranked from Easiest to Hardest to Wallop ...

D TIER: Easy Peasy:
(The Root Pack) - Once I first realized the gameplay required a full portion of attention these guys went down easily. The tears kind of messed me up but soon as I realized they moved in one row.
(Goopy Le Grande) - Easy pattern, easy to predict, easy parry, and final phase is easy once you know how to dash in time.
(Werner Wermen) - Co-oped him with a friend. Died after 5 tries.
C TIER: Not so bad:
(Cagney Carnation) - I felt stupid when I found out I could move freely when the vines wen't up, I though they would hit me like everything else. I would constantly get hit by the last pine cone during his missile attack, but quickly learned around it. The final phase of most bosses where easy once adrenaline had kicked in.
(Ribby and Croaks) - The turbine attack was hard at first until I calmed down enough to learn where the thunderclaps where going. The Bull platforms and Snake platforms are easy. The Tiger is evil.
(Hilda Berg) - The hardest part was Sagittarius with his stars while everything else was easy. And maybe the part where Taurus jukes you into getting cheap-shot-ed. Funny thing is I didn't finish her off. My buddy did.
(Captain Brineybeard) - I had trouble with him for about a week until I got the Charge and Auto Parry. He then became ridiculously easy.
(Sally Stageplay) - The more the screen cluttered the more we panicked. However the last two phases where easy, especially the last one. It took a week to fight solo, much quicker on co-op.
(Cala Maria) - The seahorse did little to hinder me, but the pufferfish combined with any other attack was an almost instant hit. Co-op helped loads. Didn't feel as rage inducing as the others for some....odd reason...
B TIER: Getting there:
(Beppi the Clown) - Of this tier he felt the most possible. The horse phase was easy since all I had to do was time my shots and stay under him. Had almost made it too the end until my buddy joined. I died but he pressed on and beat him.
(Baroness Von Bon Bon) - Despite being beaten after two nights with co-op, it felt the most grueling. The Candy Corn is the worst of the bunch while the Cupcake was pretty easy. Upon learning Bon Bon's heads are stalled in movement, it allowed up to zig zag enough to avoid them. Took a lot out of us.
(Djimmi the Great) - Cats where mostly easy except for the kittens that approached from your blind spots. The swords where hard and the jewels almost unavoidable. My buddy beat him while I wasn't watching. If you couldn't notice I'm pretty sure he's better than me.
(Grim Matchstick) - This one I feel was true for almost everyone. The first phase was easy once the fireballs can be tracked, second phase is very stressful, and the last one was impossible for me. My buddy beat him, but not before he had a fight where he got killed the second the knockout animation played.
(The Casino Mini-Bosses) The easiest combination for me was 3 - 6 - 8. I found Hopus Pocus much more rage inducing than Phear Lap, actually. But more rage inducing them Hopus Pocus was Chips Bettigan. The jump distance combined with a high up hitbox made me go oof owee ouch. Mr Wheezy is eezy. Everyone else is pretty much worth a start over.
(Phantom Express) - Once again another Co-op. Beat it in one night, but the final phase while short, was the hardest due to all the stuff flying around on screen. Also the Skeleton's head hitbox is very much too close for comfort. I'm not even going into detail about those pumpkins.
A TIER: I WANT TO STRANGLE A KITTEN:
(Wally Warbles) - I felt no remorse taking part in killing him and his son. Course all the bosses I feel where fun and had charm. But when I comes to spending two weeks with them that's where that ends. My plane was almost no match for the little bird's Egg UFO from NASA ^TM . Oh, and bullet hell.
(Rumor Honeybottoms) - TOO. MUCH. YELLOW. I don't know how many times I messed up while looking at the entire screen. That combined with the RNG made it both physically and mentally straining. I never want to see geometric shapes ever again.
(King Dice) - Three weeks of torture. Three weeks of wrong dice rolls. Three weeks of dying to Chips. So much so I'm planning a Fifty Shades of Grey erotica novel of Chips Bettigan. I dare you to make r34 of a stack of poker ships, just try me. Point is, the moment where we got enough health to finally beat him is the moment where I breathed a sign of relief. And by the skin of my teeth, too. If it wasn't for my buddy and his terrifyingly accurate rolls I would still be fighting him. Funny thing is afterwards we beat him again.
(Dr Kahl's Robot) - You ever imagine what it feels like to get punched in the balls and ovaries for four weeks straight? Say hello to Doctor Klux and the Iron Giant. I don't think I need to say much more, the name itself strikes fear into the hearts of the poor Cuphead players. The jetpacks. The lazer. The TRACKING BOMBS.
(The Devil) - Is hard as Satan. Not the longest boss, but after he becomes spooky skeleton the hits really start to show. The feeling of being so close to finishing the game yet dying so many times. It nearly put me on the brink of giving up. The blue projectiles where the worst of all of it. I fail so many times. But I always come back to it like a addict getting it's fix of cocaine, if cocaine took on a humanoid form and slapped you across the face so many times the bruises turn purple.
I'll be posting links to my death count and ranks on all the bosses. Thank you Cuphead, for teaching me fast reflex skills. I'll be sure to play it again sometime in the future. I'm not doing Expert mode. Screw that.
BONUS - Run n Guns(that have been completed so far)
Forest Follies - Easy. First thing I ever beat in the game
Treetop Trouble - Major difficulty spike. Those ladybugs throw me off track and the Big Wasp felt impossible to get to.
Funfair Fever - Haven't completed it yet. But I have a newfound hatred for balloons and trampolines.
Funhouse Frazzle - It's always that one shooting star that got me. I felt lucky completing it.
Rugged Ridge - Haven't completed it yet, but have gotten very close to.
Perilous Piers - Me and Auto scrolling do not go well. But luckily I got an OP charm (Auto Parry).