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.
Best Answer
If you were playing the PC version, you'd be correct, this is highly unusual. However, on mobile, this is intended and expected, as explained on the official wiki.