Cheese the boss.

First make sure you have a bow and lots of arrows, and food with a full belly.
Then build a double walled shack with a ladder and lookout that you can fire from.
Don't spare any wood, make it fortified with a spot to LOS the bosses lightning attacks.
You can repair the wood as he blasts it away.

Lure the boss to the shack, and fire at him until he dies.

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