To normally get these trophies, 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 play as your favorite character, and once you reach Dark Kahn, press Start, and choose "Player Select". Choose a fighter you have not completed Arcade mode with yet, and defeat Dark Kahn with him or her. Also, it is much easier to play on the Very Easy difficulty so you can defeat Dark Kahn easily with the characters you are not familiar with.