3D Dot Game Heroes

Onyx is a huge beast that has a variety of attacks. He can shoot fireballs, send out tornadoes, and strike the room with lightning. He also attacks you with his tail if you are very close to him. If you have the Hero Shield, you can block all of these attacks, so it is recommended you have it for this battle. The only attack you cannot block is a charged attack which the boss tends to use after taking significant damage. However, you can easily dodge this, as it takes several moments for the boss to charge and release this attack.

If your sword is long enough, you can simply stand at the opposite end of the room and strike Onyx from afar. Block (or dodge) whatever he sends at you, then immediately strike back. As you damage him further, he will become more aggressive with his attacks. His fireballs can be hard to block at an angle, so be careful. Overall, he's not too bad (if you have the Hero Shield).