Anticlimax Boss / Zero-Effort Boss: Banglar. He's backed up against the wall and his only attack is to fire out three easy to avoid/block bullets—and that's only if you turn your back to him. All you have to do is walk up to him and slash him to end his life. For some reason, he's immune to your shurikens—probably just to make the cutscene after you actually do kill Banglar not block out your character.
That one is the most well-known, but they're all good, you can find them all here.
Demonic Spiders: The Fire-breathing ninjas, whose fire breath can easily kill a player from full health, teleport a lot, and attempt to keep out of melee range. Crouching their fire breath will result in them crouching and attacking, making it very hard to evade damage.
Goddamned Bats: The weakest enemy, a uniformed trooper with a knife, comes at you in swarms, and are actually smart enough to crouch-attack you should you attack them while standing. Leter on they start wising up to your ninja, and will utilize flying kicks to attack you.
Tsuchigumo will keep jumping around your character, and are hard to hit. To defeat them, let them jump over you, then attack them from behind when they land.