Terraria is a difficult game, but every boss can be defeated. Except one...
The Golem is the only only Terraria Boss that is impossible to beat. Even with a Maxed out gear set, and a Zenith, and every buff in the game, you cannot defeat it. There has been somewhat effective cheeses to get it below 99% health, but even then it's quite pointless for progression. It was created as a obstacle, a force of nature, not a challenge to overcome, a being to avoid. See the Dungeon Guardian or Daytime EOL on Master Mode for a much easier fight.