Marvel Ultimate Alliance

There are two ways to unlock this achievement. The first is by legitimately playing through this entire level without killing anyone. You'll have to kill off or remove your AI characters, and then you can use a flying character to simply float to the end of the section or a character that can go invisible to sneak through. Once you exit through the door at the end the achievement will unlock.

I'd suggest doing this via the second method though, which is to simply go through the level normally (lots of enemies to throw off ledges), and then once you exit to the next section, use the doorway again to return to Cityscape. Your kill count will be wiped upon entering this area again, so just turn right back around and use the door a third time to unlock the achievement. If you've already passed this section of the game, choose the extraction point "Heights of Progress" in Act 4 to return to the door you'll need to use.