You can get this achievement by playing normally and defeating the last boss as Katarina, or by joining another player's game session just before beating the game and choosing Katarina. This is available for other characters.
You can boost a friend and offer him this achievement by creating a save just before the last fight and inviting him in your game for the final show.
Note : Those ending achievements seem to be a bit glitchy. If the achievement doesn't unlock once the ending cinematic starts, you may need to clear your xbox 360 cache and restart the game. It worked like a charm for me !