Firstly, as mentioned several times, you are way overgeared to fight Eikthyr... Generally you can fight him in little more than your starting rags, a bow with a few flame arrows and dodge his attacks by using the various standing stones as cover....

That being said... I would like to point out that there is absolutely NOTHING wrong with over-preparing, and it even becomes essential that you do (over-prepare) throughout the rest of the game, for your best chances of survival.

The first Boss is a cake walk... after that the game gets serious.