Trial and Error Based Gameplay

[Koh]

Yes.  THAT trope.  We've to talk about this.

Think about any boss ever.  In order to beat them, you have to learn their strategies, weaknesses (which entails weak points), and any thing else to successfully achieve victory.  But can this be accomplished without trial and error?  One could argue that it doesn't have to be as long as there's room for human error (life bars/multiple hits can be taken, etc.), but then it questions the design, does it not?  But one could also argue that it's inherently trial and error.

What constitutes trial and error besides being forced to do the same thing over and over again until you create the perfect run?  Is there a such thing as "good" trial and error, or is it all bad and frustrating?