Can I kill the Golem pre-hardmode?
Thank you for tagging Terraria mobile instead of PC, as you can break through with Dynamite in Mobile.
Now, to answer your question, you should prepare both with gear and with a plan.
Weapons
For weapons, I'd say it depends on what you're better with, though ranged/magic damage, to me, is the better choice. If you're into melee, use the Night's Edge, crafted with Blade of Grass (crafted with UG Jungle materials), Fiery Greatsword (crafted with 20 Hellstone bars), Muramasa (found in the Dungeon), and the Blood Butcherer/ Light's Bane (crafted with Crimtane/ Demonite bars, respectively). As for Magic, use weapons that can go through walls. If not, and you want to use guns, use the Minishark.
The Arena
Since you're on pre-hardmode, I would assume you haven't beaten Golem before. Therefore, you can't break the bricks in the temple. What I did was place blocks above the altar, exactly where the head can be above, but the body is below. I also put two wood platforms at the right-most area so I can easily jump up to the section I needed. I placed 2 blocks of honey and made a pool on the left, and on the right I had a chest with a lot of Healing Potions in it. However, your temple may vary from mine so you may want to set yours up differently.
Accessories, Armor, and More
First off, you need to be able to move. Things like Hermes Boots, Sandstorm in a Bottle, or Tiger Climbing Gear will help. You also need protection, like a Cobalt Shield or a Shackle. Any other accessories you may want you can use. As for armor, you definitely need full Molten Armor, made from Hellstone bars. Lastly, get all the healing potions you can find.
Further Advice for Preparation
Reforge. Reforge, Reforge, Reforge. I can assure you the most helpful NPC you can get is the Goblin Tinkerer. Now, you're gonna need a lot of coins to get the prefixes you want, so those will be helpful.
The Battle: 1st Phase
The Battle begins as soon as you use the Power Cell. If you did the Arena how I did, run to the right and jump up the platforms. Pull out a magic wall-piercing weapon and shoot him. The fists cannot hit you but the head can, so at the top right I set a platform where he can't hit me. The head shoots fiery balls of death, which will be blocked by the platform. Keep shooting him until his head is at half health. Then, he will begin shooting wall-piercing lasers, from there, you will need to keep moving and make sure not to get hit, as when the lasers hit you they will take off a large amount of health. Keep dodging the lasers and hitting his head with the weapons. Once that gets to 0 he will enter the second phase.
Phase 2
His stone head will completely detach from his body, revealing a mechanical head that will attack you as well. The stone head can be destroyed, but you have to destroy the mechanical head. The mechanical head will be harder than the stone counterpart, so mostly just keep up what you were doing since none of the other body parts will attack you (with the exception of the stone head). Once the mechanical head gets down to 0, you beat the boss and all of the parts die. Good job!
Hope that helped... sorry if it's long.
It's not possible to kill Golem pre-hardmode because you can't fight it pre-hardmode.
This is because to to be able to enter the Lihzahrd Temple where the Lihzahrd Altar is you need the Temple Key which is dropped by Plantera, to which it's bulbs which when destroyed summon it only appear when the 3 Mechanical Bosses have been defeated.
To summon the Mechanical Bosses you either need their items which you need Soul to craft which only appear in Hardmode or destroy Demon/Crimson Alters with the Pawnhammer which is only dropped by the Wall of Flesh when it is killed, killing it puts you into hardmode.
Ofcause the above is the Legitimate way to summon it. Under the notes of the Golem Page
- The Golem can only be summoned in a world where Plantera has been defeated. Attempting to summon it in a world where Plantera has not yet been defeated will fail, and a Lihzahrd Power Cell will not be consumed from your inventory.
In order to use a Lihzahrd Power Cell, you must right-click the altar. Holding a Power Cell and left-clicking will do nothing.
- Like most bosses, Golem will despawn if the player travels too far away, including traveling back through the Lihzarhd Temple.
Golem currently does not drop any coins, the only reason you should farm him is for his drops.
It is possible to summon Golem pre-Plantera by using the Power Cell when you're under the room's floor.
As you can see there methods to get to it pre-hardmode or pre-Plantera in Console or Mobile Version only. however it should be noted that in the History, version 1.2.3 is when the Golem is unable to be summoned unless Plantera is defeated in that world.
the current Mobile Version is Version 1.2 equivalent on PC with a number of hot fixes for Android
the current Console Version is Version 1.2.2 PC equivalent
as such when Console and Mobile Version are updated with the 1.2.3 PC equivalent Version it is more than likely that these methods will become invalid unless there is a method to summon Plantera illegitimately pre-hardmode