I love to use Piranas and Nightmare, but you are a senior, so I can't help you that much.
Here's what I use:
Nightmare: Ambre; Estalt; Ghumbo; Kenny; Kolos; Nistarok; Oshitsune; Vince.
Piranas: Ambre; Bloodh; Dalhia; Katan; Pesth; Scubb; Selma; Sliman.
If you would write what cards you have, i would be glad to help you with building a Nightmare deck.
first piece of advice: Sargh and Erzsebeth are two cheap cards that will help you win rounds without outpilling your opponent, something you need in a Nightmare deck, as your power is generally not that great.
Mojo is a good supplement, to begin with, because your lack of dammage forces you to considder lifegain abilities, later however you would do well to find a card with higher power to replace her.