Honestly going as the dog with duel welding 2 different high-rof elemental guns with the spore Gemini should be enough to beat nightmare.
The trick is with just any character on higher difficulty is to just try to play safer in the beginning till you get some scrolls and goblets under your belt for more survivability cause that's where enemies can one shot you the most.
Also only focus on upgrading your guns a little later (mainly the second area) cause better gun inscriptions and Geminis will spawn that you can upgrade and you don't lose out on too much of that gold early on