Partway through Contr Abobo, Kirby kills Krang and hijacks his mecha suit, turning it into a bigger, better version of itself. Four years later, and...
After you knock Little Mac down twice, he Hulks Out. He'd later do that for real in the Wii iteration of his home game, and later get that form as his Final Smash.
I Am Not Shazam: The naming of the enemies (who are almost all directly lifted from the NES games) is incredibly inconsistent. Some of them retain the original names from the titles they appeared in, others have made-up names, and others take the name of the game they starred in, such as Kid Icarus and the Balloon Fight guy. Granted, since Captain N: The Game Master shows up in the ending, it's probably less a major case of I Am Not Shazam and more of a sendout to the Captain N cartoon, which infamously referred to Pit as "Kid Icarus".
That One Boss: The Old Man, the boss of Zeldabobo. By the time you first enter his boss room, you're probably hurting for health. He starts off with a shield, and he kicks the fight off by spawning enemies you've fought earlier in the stage (which means he can even spawn Flurries, the Goddamned Bats of the dungeon). He can send out a homing projectile which, if you get hit by it, will be turned into a snail for about ten seconds, and will be more prone to being attacked. He can fire lasers and a homing stream of fireballs, which can also be frustrating, even more so if you get hit by the snail spell at a wrong time. Then, after taking enough damage, he splits himself into three more old men, who can use a spin attack, which means more damage if you don't see it coming (but thankfully, if you have enough rage at this point, a single rage attack will end this phase of the fight quickly). THEN, he drinks a potion, transforming into the Old Giant, who hits like a tank. This portion of the fight is considerably easier, since he's a much larger target and his attacks are easier to dodge (unless you get unlucky), but he gains invincibility frames during his attack start up, so this fight is a lot harder than it needs to be. The cutscene afterwards does make up for it, though.
That One Level: Pro Wrabobo, as stated on the main page. It's not because of the boss portion that makes it frustrating, it's what comes before it that gives players a hard time. Getting through the Balloon Fighter part can be easier said than done, as there's a ridiculous amount of obstacles between you and the boss fight, the Amazon keeps popping in to shoot a spread shot at you constantly, and, if you're trying for the two "no damage" achievements for this portion, expect to get pretty frustrated.
Ugly Cute: Abobo in Zeldabobo, the first part of Pro Wrabobo, and Mega Mabobo.
Aboboy, to some extent, and The Amazon's sprite, during the Balloon Fight portion of level 5.