Golem

The Golem is a Hardmode boss and always drops 1 of its 8 items.

It is found inside the underground Jungle Temple, which you can access after acquiring the key from killing the new hardmode boss Plantera. After obtaining the Picksaw from Golem, you can access the dungeon without a Temple Key.

The Golem is summoned by putting Lihzahrd Power Cells into the Lihzahrd Altar. There are 3-14 power cells per jungle dungeon, varying on the size of world you created.

Once summoned, the Golem has 15000 health, and begins to attack. In his first phase, his head will spew fireballs every 2 or so seconds, while the hands extend towards you and deal damage. It will be somewhat mobile during this phase and you will be able to easily outrun it. At the second stage, at 7500/15000 HP, the Golem will turn its head towards you and start belching faster fireballs as well as shooting yellow lasers. Its jumping and arm extension will also be faster the lower its HP is. Interestingly, when the Golem starts shooting lasers during this stage, being exactly above it (so that it faces forward), it will only spew fireballs and not lasers.

Once downed from 15k HP, the Golem will separate, the head floating above the body. The head will become invincible and spew fireballs similar to phase 1, and lasers when the body becomes moderately damaged. At this time, the body of the Golem begins to jump towards you, and has 8000 HP. It is very difficult, if not impossible, to evade the damage from the body at this phase. As in the first phase, it will jump further and more often the more damage it takes.

Strategy
There is an easy way in killing Golem by building two boxes (rooms) around the altar. One box is where the head will spawn and the other around the altar where the body spawns. When the Golem spawns the head tries to fly towards the body. But since it's trapped inside a wall it can't and will be stuck. The main body of the Golem will be also stuck inside the second box. Adding fire and spike ball traps from earlier in the jungle temple can greatly increase the effectiveness of this method, allowing players to kill the Golem in a very short time with minimal danger. By making small holes inside the walls you will be able to shoot through and hit the Golem. The best way is to kill the head first then after that kill the main body and ignoring the fists. Since the Golem is trapped inside the box it won't be able to jump on you or hit you with his fists.

Another easy way to defeat the golem is to get the Vampire knives and high-tier melee armor (Hallowed or above). Then simply stand under the Golem and keep throwing the knives. If your melee damage is high enough, you will recover health faster than he damages you.

Another fairly easy way to defeat Golem is to build several layers of wooden platforms over the Altar. When the Golem moves to jump-stomp, it should get trapped on the upper platforms, allowing you to attack it with ease, taking minimal damage from the fireballs.

Bug
The altar in which you spawn the Golem with has a chance to spawn in the ceiling. This causes the Golem to be summoned stuck in the ceiling. The body might slip out from the tiles, but the head is forever doomed to linger in them making him unkillable, assuming you don't already have a Vilethorn, Nettle Burst, or the Picksaw which could simply just mine to the head. A Chlorophyte Partisan can also be useful if the Golem gets stuck.

Sometimes, if the Golem is far from the player, it will despawn.

Another bug is after killing the Golem, it is possible for all bosses to spawn at night, every single night or one every once a night. All may spawn at once during the night or just one.

Sometimes the way to the boss is blocked by generated blocks, preventing you to kill the boss. If you have never killed him before you can not enter at all.

It's possible to spawn the Golem on a non-Hardmode world, as there's no check against it.