Nightmare is really hard... I don't think you can solo it. And this map takes forever on ANY difficulty level.
You basically need at least 1 person to take care of the base because Wyverns and Ogres are problematic.
I recommend a Faerie for any character outside of the base.
I think one of the best setups is probably a 3man party with 1 base camper, 1 person grabbing a male, and the 3rd grabbing the corresponding female and have them meet on the top bridge in the middle.
I was able to solo this map on Medium, and 2man this map on Insane without much difficulty. Just needed to keep an eye on repairs for when you have mass Wyverns and multiple Ogres in the later rounds.
Our simple strat was basically to spend a massive amount of time just building and upgrading everything to the max in the first 2 rounds. You don't even need a Genie for this because the monsters are infinite spawn, but I recommend a Genie for at least the first round anyway so that it goes faster.
If I remember correctly there aren't any major rewards for beating it on Nightmare...? I think you can unlock all the skins just from Insane mode. You only need Medium mode for the Achievement. There isn't really any reason to play Nightmare unless you want a high statted pet I think.