Too Human

The quickest class to probably play and clear this is a Commander and made sure to get the plasma turret spider (great for crowd control). Guns seem to be a powerful weapon due to their rapid fire nature versus melee combat.

After the first level, you will be required to report to the Armory. Once there you will be given the choice to become a Cybernetic or Human -- for this achievement choose Human.

Here is the drag, you cannot jump into another player's co-op game and play the final second (Helheim 4-5) once you've made your selection. You are required to roll through the entire campaign in order to unlock this achievement.

Playing as a Commander on my second true playthrough I was able to get this achievement in under 8 hours with about 25 deaths. Just don't get side tracked and continue along the beaten path and this achievement will be yours fairly easily.

At the end of Act 1, you will go to the armor room, and there you will be given a choice of becoming cybernetic or human. Both of these will give you access to two new skill trees with new abilities on them.

Once you have chosen Human Baldur, play the game through to the end. When you have defeated Hel, the achievement will unlock.

(FYI-I used Too Human 360 achievement guide Credit for this solution goes to Hayden)
http://www.xbox360achievements.org/forum/showthread.php?t=88...

(before you give me a "thumbs down" on this or any other solution. Tell what I can do to fix it)