This Achievement is for Mastering 50 skills with Eleanor. This achievement is simple. Each piece of equipment has a skill that can be mastered after so many battles wearing it. Simply wear an item for as long as it takes to master it and then equip another item and master that one. There are 5 Equipment slots(Weapon\Accessory\Armor\Ring\Boots) so you can be attempting to master 5 pieces of equipment at once. This can easily be obtained in one play-through if you start this from the start.