Pikmin 4 Boss 9: Groovy Longlegs 

The MLG airhorn. This really is a world lost to time.
Difficulty: 9/10
Now yes Purple pikmin and red potions can make this fight much faster but let's assume for a moment you don't want to waste what few purple pikmin you do have in this death trap of a dungeon. And I'm not using red potions for any major bosses, if it's a miniboss you'll see dozens of like Peckish Aristocrab then sure but Foolix, Water Wraith, and Groovy have all been major fights and I don't wanna just show cheese.
That gives us two ways to fight this boss. Quick and reckless or slow and safe. Winged Pikmin will relatively safe from his crushing attacks, and if you had rocks those would probably break this fight. But winged pikmin have a .75 damage multiplier meaning they're weaker than any other pikmin type in combat and you only have 10 of them max.
This boss has 3 forms it shifts between and two separate phases.
In Phase 1 he'll have his slow and steady mode where it kinda seems like he targets you. Just be careful and throw some pikmin in when you can. Getting on Oatchi allows you to have a smaller target area but also is a bigger risk if he hits the pikmin, in fact this fight took me 3 attempts, attempt 1 he killed me outright, attempt 2 he killed all my winged pikmin in a single shot causing a pikmin extinction (sort of it'll have one if all of one type get wiped out) then there's this fight where Oatchi ran out of health.
Anyways after taking damage he'll either try to throw you off like Beaty Longlegs or blast some rave gas. Anyone afflicted by the rave gas will start dancing under the disco and you can't whistle them out.
Phase 2 is where the real danger begins, because he can engage yellow mode on his own. He can also let out some groove gas multiple times and then enter his red Rave mode which may as well be a blender for how deadly it is, if you have only one or two pikmin afflicted by the gas you should be ok but if it's over 5 at least one of them is going to die. It also has a new attack where it rears up and slams down 2 legs at once. This is very dangerous because not only can it crush a lot of pikmin but sends out a massive shockwave that will send everyone flying and more than likely multiple pikmin will get sent off the edge, seriously they go flying across half the arena.
But I still think this fight is awesome, it's got the dynamic boss music, a cool design, and brings some significant challenge and even with red potions this boss won't be bursted down in 10 seconds since you can't really charge pikmin onto him unless he stops to rest and that's often bait for rave gas.
So yeah fun boss and now we've found all the the missing students the professor mentioned, the professor is in Engulfed Castle, 2 students are in the dungeon with the big wollywog (one was swallowed by a greater spotted jellyfloat while the other is in the boss room) and now we've found the third and final student she mentioned.