Too Human

This Achievement is unlocked when you assign your first Skill Point to the Sentient Ability node on the skill tree. This is the bottom-most node on the skill tree and enables you to send your melee weapon out on its own to fight enemies by clicking both stick buttons with a maximum combo. This frees you up to simultaneously attack enemies with your ranged weapons.

Depending on how you have been spending your skill points, you should expect to unlock this somewhere between level 17-19.

Sentient abilities are unlocked by reaching the bottom of your skill tree. Once you reach the bottom, place 1 skill point in the tree to unlock this achievement. You will need at least 35 skill points to reach the bottom of the skill tree to unlock a sentient ability.

An indicator will appear under the word ammo on your HUD in the shape of a wolf, indicating that your ability is available. To activate your sentient ability, you need to click in "Left and Right thumb sticks" at the same time.

(FYI-I used the 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)