Dark Guardian

Dark Guardian resembles a giant Nightmare that uses a hammer imbued with the power of thunder. He is the temple boss of Shamar in the game Sonic Unleashed. In the 360/PS3 version, it is mentioned that he was created when the Shamaran temple absorbed moonlight.

Boss Guide
The strategy for this boss is completely different between the two versions.

Wii/PS2
This battle is like fighting a regular enemy (like a Titan). You simply hit him while avoiding his punch and hammer attacks. After a while, he'll teleport to the other end of the arena and slam his hammer causing lightning to strike various parts of the field. If you hit him before he finishes the attack, this will trigger a quick time event where Dark Guardian will fly at you to punch you and you must press a button to stop him and knock him over causing a little bit of damage.

360/PS3
The arena is between the Sun and Moon altars and you must use them to your advantage. Hit Dark Guardian a couple of times to stun him, then push crates over to the Moon altar to destroy it, which in turn causes damage to Dark Guardian. There are several obstacles preventing you from doing this. First, spiked fences randomly pop up and down, blocking your path. Second, Nightmares will spawn and try to hit you or push the crates the other way. Also, Dark Guardian will try to hit you or summon lighting (which is why it's recommended you take him out first). If part of the altar is destroyed and he is still conscious, he will rush at you and try to punch you. You will be prompted to press a button to block the attack and knock him over. Do this three times, then hit him a few more times to finish him off.

Trivia

 * In the 360/PS3 versions, when you beat him, Sonic makes a reference to Amy: "I know little girls who are better with a hammer than you. Well, one, anyway."