If you get killed by Eikthyr then you were massively not prepared.

You should have:
1. A complete set of level 3 deer armor (lvl 2 might work in a pinch).
2. A level 3 wooden shield.
3. A level 3 flint axe (or other weapon of your choice).
4. Be fully rested, so you don't run out of stamina too quickly.
5. A full stomach, preferably on deer, boar and honey.

You need to:
1. raise your shield when Eikthyr attacks and tank the hit,
2. attack Eikthyr when he's not attacking.

Eikthyr should then be easy to kill.

In regards to your problem getting killed over and over... did you build your shelter close to Eikthyr's altar? Probably a bad idea, as you now found out.

I would move to another place on your starter continent and start over. Shouldn't take too long. Once you have the required prerequisites, you should be able to easily defeat Eikthyr and then you can move back to your old place.

Eikthyr is by far the easiest boss, and is there to teach you the boss mechanics of this game.

Good luck, you can do it!

Edit: Just to hammer this in a bit more: Having a full stomach on the best food you can get is CRUCIAL in surviving this game.