The Legend of Zelda: A Link to the Past

Do you mean the boss? If so, you need to hit its mask with the hammer or with bombs, once the mask is destroyed, hit the green thing in its forehead with your sword or with arrows.

The boss' name is the Helmasaur King. You can destroy his mask with either the Magic Mallet (found in the dungeon) or bombs. If his tail starts swinging back and forth at an increasing rate, try to stand directly in front of him closer to him and the tail will pass around you. Also, look out for his splitting fireballs. Once his mask breaks, it will expose a green gem on his forehead, his weak point. Next, you should try using arrows to hit his gem. However, if you're out of arrows, you sword will suffice, although it exposes Link to more damage. Good luck, and you should be at the next dungeon and no time.

All you have to do is break it's mask with the magic hammer and then attack it with your sword.