To obtain Power Hungry you must defeat 100 enemies using power attack.
Power attack is a 'rage' esque attack, unlocked from the merchant. Once unlocked, your power meter will fill. Wen full, hit B to activate it and kill as many enemies as you can before the meter empties.
I would heavily recommend unlocking power attack ASAP, as these are the kill achievements that will take the longest.
Upgrading the following will help speed this along :
Longer power meter
Fill power meter based on damage given
Fill power meter based on damage taken
If you upgrade these three fully, you will get to use power attack fairly often, and make this much easier and and faster.