Fools idol (the boss you describe) is overcome by exploring the level and noticing the dude saying "i wont interfere i promise" who is right next to the boss. If you are impatient and go straight to the boss without exploring you are punished, but then it lets you escape the fight after the realisation that the boss is immortal so... not that unfair.
It does have it's moments where it requires trial and error, but it overcomes them (for the most part, old hero is an exception to this) with clever level design and tons of shortcuts.
When you complete the game you go to "new game +" and can replay the game with all your epic gear so you can easily get anything you missed the first time (like the alternate ending, and numerous "tendency events" that are hard to get in 1 play through).
It's quite apparent, but i'll say it anyway - i am a huge DS nerd ^_^. I'll admit it's not everyone's idea of fun but for what it aspires to be, it's a very good game.