Golem

The Golem is a Hardmode boss. It is summoned by using a Lihzahrd Power Cell on the Lihzahrd Altar, which is found in the Lihzahrd.

Like many other bosses, the Golem has two stages. In its first stage, the Golem will shoot fireballs, extend its fists, and occasionally jump around. When the head reaches below half health, Golem will begin to shoot lasers.

Its second stage activates when its head has been "killed". Once this stage activates, the Golem's head will detach and fly around above its body, taking no damage and firing lasers at the player whilst they continue their fight with the body. In addition, its movement will speed up.

Tips
Utilizing this same mechanic, occasionally the altar spawn in the tunnel just before the large temple room. Most, if not all, portions of the boss will be trapped in the brick around it, making the boss fight either extremely easy (If you have a weapon that passes through solid blocks) or impossible (if you don't).
 * Being one of the two bosses with a collision box for blocks (the other being King Slime), it is possible to beat Golem without incredibly good gear. While exploring the Lihzahrd Dungeon gather all the traps. Construct vertical pillars out of traps large enough for Golem but high enough for him to not get out on both sides of the altar. Finally line the ceiling with spike ball traps. After summoning Golem leave the enclosure and repeatedly activate traps to kill it.
 * You can place platforms in the room vertical wise and the Golem will spawn on top of the platforms making him easier to dodge and attack from below.
 * The easiest way to kill the boss without any problems is to make some platforms above so the Golem will spawn on them, and then hide under a two block wide ceiling to avoid getting hit by the flame balls. Then using the Chlorophyte Bullets to simple shoot "around" that defense ceiling to hit the Golem. When he starts to fire lasers, ignore that, just stay there and keep shooting. He will be dead before your health reached the half.
 * Another way to kill it easily is to make a roof over an edge of the room, wait until he jumps on it, and start attacking him either with a mushroom spear or the chlorophyte swords. Since the floating mushrooms/ poison cloud go through walls, he cannot damage you until the second stage, and even then only the lasers will hit you. If you don't have that much defense, try getting a cross necklace for the longer invincibility time.
 * Megashark or Stynger ranged weapons can easily destroy this boss in record times. Though you can only get the Stynger from the Golem itself.
 * At low health, the Golem will begin firing lasers faster, so ensure that you have a suitable number of potions. It will also extend its fists more frequently. Try to destroy these first so that they don't become a problem later.
 * If you manage to get a Piranha Gun, it is very easy to slowly take out the Golem by hiding after shooting.
 * Using the Bunny Cannon and a wall between you and the Golem, you can very easily take this boss out in a short amount of time. This takes about 50 explosive bunnies, just spam-click the cannon until it dies (tip: aim the cannon straight up and wait for the boss to stand on top of it).
 * The Golem only fires lasers during two parts of the fight, first when the head reaches 7500HP, after the head comes off and the body becomes vulnerable, the head will stop shooting lasers until the body reaches 4000HP, in which it will shoot lasers again.
 * Another way to easily kill Golem is to make a roof above him with blocks that don't fall, like dirt blocks, and right before spawning him, place 2 clouds with the Nimbus Rod below the roof. Then you must rapidly move over the roof of blocks, from there, you can keep placing clouds as their rain will go through the blocks if they are close enough to them. A Spectre Armor set and a Cobalt, Obsidian or Ankh Shield will greatly help for this method.

Trivia

 * The Golem might be a reference to Kaos Karnage from Donkey Kong Country 3: Dixie Kong's Double Trouble
 * This is commonly thought of as a hard boss, but can be defeated easily with the right preparation.