Having the Lightning Rod (single target) and Updraft Tome (AOE) makes it easy to take down Redstone Golems in the game.

I for one am a big fan of the Lightning Rod and use it as much as possible in any Minecraft Dungeon build I make. It is in my experience, one of the best sources of single target damage in the game. And since it has a very low cool-down, you can use it frequently against tough enemies.

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Me and my son defeated the Tempest Golem a few months ago (this game log is months behind). The key was to focus on clearing out one zone at a time. There are two of them in the boss fight against the Tempest Golem. And it also helps to have lots of AOE damaging artifacts, like the aforementioned Updraft Tome, and some other ones like the Harvester and Satchel of Elements. Once you clear out the two zones that spawn enemies, the Tempest Golem will be vulnerable to attacks.