I generally get done once or twice by the bosses while I learn their attack patterns. There's been one exception, and there were...extenuating circumstances, let's say.

Bizarrely, the first boss was dead easy on my first couple runs through him, but on my third run he suddenly went nuts on me and wiped the floor with me over and over again. I eventually gave up and went to smack my way through a different area instead.

Also, fire sword vs. fire boss is hilariously ineffective. I hit it once, said "I'm dead." and resigned myself to watching his attack pattern for the rest of that life.