Atgeir is pretty good at blocking stuff. It, of course, can't completely replace a shield.

But that's actually not the point of the atgeir. With the atgeir, you are trying NOT to get hit. It has long reach, so most of the time, you can stop smaller enemies before they are in melee range.

Bigger enemies can usually be stunned with the secondary attack, which, IMHO, is almost too effective, because once stunned, enemies can be very quickly defeated, as you crit most of the time.

So you need to completely change the way you're playing. From a melee oriented tank to a damage oriented melee heavy or ranged medium fighter.

Your armor could change to medium or light. For example, I recommend root armor for plains due to its excellent pierce protection. You just shrug off Deathsquitos or fuling spear throwers.

All other enemies should be kept at range. In conjunction with a good bow (and the added bowskill of the root armor), the atgeir becomes a very effective weapon.

You would also change your food layout from HHS to HSS (two health, one stamina to one health, two stamina), because you want that massive stamina to be able to not only run away, but to employ the secondary attack of the atgeir effectfully, as well as using the bow, which drains a lot of stamina.

So, yes, absolutely possible, but you need to change your way of playing. You need to be quick on your feet and you will be even more cautious, as you ab-so-lute-ly DON'T want to get hit!

Edit: You absolutely want the Bonemass forsaken power when playing like this in case things go south. Or you could still employ Eikthyr's power to run away even farther. :D