The fucking soccer ball boss in Wario Land 2 (or was it 3? I can't remember)
Completely unintuitive and when I finally did it it just felt like random chance.
So, Shoot from Wario Land 3 then? Cause B Bunny was arguably a lot easier.
In wario land shake it, rodrick, rullrant, large fry cook, king shake it, plant thingy
In other words, all of them. You're not the only one to think that. Shake It's bosses were way, way more difficult than the rest of the game.
On another note... I do have some choices for hard bosses myself:
The Shake King. Absolutely insane to be honest, puts up the kind of fight Bowser only wishes he could. Look at this:
That's just brutal, especially when you're trying to avoid taking a single hit. And the timer ticks down in the cutscenes, which is rather cheap.
The Spiked Koopa in Wario Land 1. Not too hard, but for a boss in a game as easy as this one, he was quite the surprise difficulty spike (no pun intended). An obtuse weakness (if you're stuck as Small Wario), about four attacks and different phases and a shell charge that's ridiculously accurate in following Wario... that makes for a rather awkward first boss, especially when the next four or so get steamrolled into oblivion in about four seconds.
The ghost in 1 as well. Not hard, but annoying, given how one touch freezes you on the spot. If you get hit by this, you are taking damage, no questions asked, since the ghosts he fights with get a free hit in.
Spoiled Rotten in Super Hard mode. 15 seconds. Better hope you're very, very quick!
Cractus, from the same game. Very fun to fight, very cool looking... probably the hardest boss in the game bar the final one. Cuckoo Condor and Catbat are wimps (fun to kill, but still wimps). Aerodent is just annoying. Cractus is potentially hard, especially for a new player.