Um...HAG1 is incredibly easy. And it's from Tooie, not Kazooie. Clockwork Kazooie eggs are your friend...in fact it can't even take very many clockwork egg hits.
The hardest boss I've encountered? Probably one of the bosses from Ikaruga. The third boss, I suppose, just because I still kinda suck at the timing of that one.
Also, in before ultra-famous video of final boss from Mushihime-sama Futari.
22 medium-jumbo frets, 3 mini humbuckers, 2 single-coil pickups, and 6 steel strings of pure sex.