Have you done any survivals? If you aren't familiar, you can do a survival on any map and it goes all the way up to wave 25, plus you can replay the map from the last wave you beat (up to wave 23, excluding wave 14 because that's where a pet drops). Trust me, it might be a bit boring to play through wave 1-5 again but you'll only have to do it once and it gets much harder real fast. Also, the gear and exp rewards are way better than campaign, which is helpful to prepare for nightmare, because nightmare has a level requirement for a reason (for reference, deeper wells wave 1 on nightmare has a