Before you enter a level, bring about five or six Sticky Bombs with you. Save them until the Boss fight. Once you fight him, back up and use the Sticky Bombs. It should only take a few. This is the easy way to defeat Bosses, as the Sticky Bombs take off a lot of life and are unblockable.

Use a Blow Gun Dart to stun the Boss (or a Smokebomb if you are out). Then, use a Fire Dart while the Boss is stunned. If you have enough, the Boss will die before reaching you. This works well on Tenrai. Note: Save your items for only the fights that are difficult.