To get either of these achievements, you must complete the game with all characters in Arcade mode from each side. If you do not want to play as all the characters, you can simply select your favorite character and once you get to Dark Kahn, press Start and go to "Player Select". Choose a fighter that has not completed Arcade mode yet with and defeat Dark Kahn with him or her. Also it is much easier to play under the Very Easy difficulty setting so you can defeat Dark Kahn easily with the character you do not know how to play as.