The battle was epic. It took two in-game days.
The alter I found was in a small depressed cliff area surrounded by large rock outcroppings. I thought I’d be slick and create hidey holes under/in the rocks. I dug out three hideouts where I set up extra arrows, a portal, etc.
Yeah, that wasn’t smart. Modor turned them all to rubble. We’ll mostly. My portal hole survived. I kept hitting it with fire arrows and every now and then I’d rush in and whack it with the sword.
Night fell and the wolves were coming out so I ran back to the portal and went home. Took a nap, had a bite to eat. Made some more arrows. The next morning I went back and found a golem. So i tried leading it back to Modor. It worked. Modor laughed. The golem laughed. I laughed. The golem died. I ran.
Anyway, about 100 arrows later I finally killed Modor. Now I can start setting up for the plains.
Yay.
EDIT: OH MY GOD the amount of rock, wood, and silver that I was able to pick up in the aftermath!
Source: https://www.reddit.com/r/valheim/comments/y4yzwf/i_soloed_modor/