You will unlock your 'Panther Power' after defeating Xi Wang Mu. This power summons a phantom panther which will run across the screen hitting every enemy in a line, getting stronger with each consecutive hit. To summon the panther you must have enough energy then press +. Simply kill twenty enemies using the panther to unlock the achievement. A great place to unlock this is as soon as you unlock it, as you will conveniently find multiple enemies lined up in a row.