No it's the one where there's this secret passage above a bookcase that you have to wall-kick to access. Then it leads to the balcony where there's a massive ghost.
It kills you in two hits and there's strawberry float all space to manoeuvre, so there's a big risk of falling off if you try the usual running nosedive technique. If you fall off, then at worse you lose loads of energy or die and at best, the boss is reset. That happened to me plenty of times. Even if you don't fall off, using the dive means it takes ages to get up, usually resulting in the ghost hitting you.
Also, sometimes, when obscured from view by the ghost, Mario would accidentally go back through the door, which would also reset the boss. But the most annoying thing is how the collision detection is off, so even when you're blatantly punching it, nothing happens. So desperately, you mash the button (because you can't just run away, as that will result in the ghost hitting you because he's gone opaque), which causes the 3-hit sequence, but it's futile because you know that as soon as the sequence ends, the ghost is going to kill you.
I finally did it the other night, after about 20 stressful attempts, then I turned it on the next day to discover that the emulator hadn't saved for some reason (despite making SURE to choose "Save And Continue"). But I thought that, as I'd done it once, the next time wouldn't be so bad - but I was wrong. After the 2nd failed attempt I felt my heart rate go through the roof and my hands started shaking but I carried on to no avail. After loads more attempts, I just lost my rag and smashed the pad. This was after punching myself in the thigh really hard and nearly breaking my laptop over my knee.
All in all, it reminded me why I stopped playing computer games in the first place. They really are bad for my health...
The best way to kill the ghosts is to face away, wait until they're right up your arse then do a back flip / ground pound combo. Never fails.