In any game where there's something to kill, use a RAM editor such as cheat engine or hex edit, find that killable thing's: ram address and value, using it find its health pointer, reload the level, set the pointer value to 0 as a constant before you come close enough to start combat or see it in the distance, proceed to explain what happened here.
For more interesting results choose a game where killing that opponent triggers an event, preferably cutscene, loading screen or some other visible change.
If you are still bored, find a call function or code (sometimes assembler) that triggers that event (quest finish), learn how to resolve it, try to end the game without killing anything skipping all events like that.
