Bone Lee

Bone Lee is an uncommon enemy found in the Hardmode Dungeon after Plantera has been killed. It is incredibly fast and deals large amounts of damage. They are immune to the Poisoned and Venom debuffs. They also have great knockback resistance, which makes it a formidable opponent to fight.

Bone Lee and other dungeon hardmode enemies only spawn once the underground jungle boss Plantera has been defeated.

Tips
One effective way to fighting Bone Lee is to have a fast attacking melee weapon with good knockback. This is a great spot to make use of the the Mechanical Glove and the Golem Fist. Having knockback immunity (by grappling onto the floor or using some form of a Cobalt Shield) also seems to be a great help. With its fast moves, high attack damage, high defense, and one thousand health, it should be treated somewhat like a boss.

If you want to take a magic approach to beating Bone Lee, your best option is a Magnet Sphere. The rapid attacks from the sphere will all but immobilize him. Combined with good mage armor, you can kill him with one cast.

Though Bone Lee is good at jumping, it cannot fly. As such you may try - if there's enough room - to make yourself unreachable by standing on a platform (which there are plenty of in the dungeon) or use an item (like wings) to reach a high-up location or the roof. In the former case you may have to destroy other platforms to prevent Bone Lee from climbing up to you, and in the latter you may need a grappling hook. Either way, once outside of Bone Lee's reach you can use magic or a ranged weapon to easily kill it. Keep in mind however that several other enemies will have no problem reaching you.

If you take him to the surface he will run away.

Trivia

 * Bone Lee's name, appearance, and attacks are all based on real-life martial artist and actor Bruce Lee.
 * Bone Lee may also be a reference to "Rock Lee" from the Naruto anime.