Risk of Rain 2

This could potentially be one of the harder achievements in the game, since it says without getting hit, not without taking damage. However, if you want to make this achievement guaranteed, you could play in co-op. Simply don't exit your drop pod when you land on the map, and let your co-op partner activate the teleporter and charge it, all while you are invulnerable inside your pod. Once it's done charging, you'll unlock this achievement too.

This achievement certainly doesn't require co-op though, as I unlocked it naturally. While you may think this is easier to do in the first stage or two, because there are less enemies and they aren't as strong, that's actually not likely to be the case, because you won't have a solid build in place yet. I unlocked this achievement multiple stages into a run, where I was so powerful that I was killing bosses and enemies nearly instantly.

This is definitely easiest to do "legitimately" (i.e. solo) with the Engineer, both because you can drop your two turrets and let them do all the work, and because you can drop your bubble shield around you to make it impossible for any projectiles to hit you. It's mainly the first ten seconds or so after activating the teleporter where you're at risk of getting hit, since the boss will spawn, as well as some elites or bigger enemies, and big mobs. Once you wipe out the boss and main enemies, it just becomes a matter of waiting for the charge to finish, and your turrets should easily wipe out and stragglers that spawn in. For example, the clip below shows me defeating two Wandering Vagrants in under five seconds for a challenge for the Engineer, and I didn't get hit at all during this teleporter charge. My turrets just destroyed everything that spawned, before it had a chance to hit me. Don't forget to use your bubble shield in maps like this where the enemies are practically all ranged enemies.

https://www.xbox.com/play/media/CWVTSJeC2m