If you really want to combat Nightmare make a deck which has good abilitys and a dr/stop opp ability ability if you are playing in a mode with kolos.
If you can't beat them, join them.
Seriously, playing a faction is the best to learn its strengths and weaknesses.
Because SoB clans are great equalizers (They remove any synergy you have on bonus/abilities) - and they have decent abilities to give a fight there is one way to beat SoB clans consistentlty:
Rainbow decks. Just pick the most ridiculous abilities and bare stats from the field and youre done and ignore the clan factor totally. Also there are just 2 SoA's on Nightmare and only Glorg is the dangerous one so you can afford to have cards that lean mostly to their ability. Copper, Askai, Diana, Louise etc etc etc...
To counter the nasty power reducers (Eadh, Edwin, Oshitsune) simply use something that pulls those even lower - again ignore the clan bonus totally ... Trish and Havok will shred all those cards to pieces and these are the best counters because of the power difference (Trish against Oshitsune = Instawin.). Or use high profile SoA cards like Jackie and Hawk - Or use the protect power ability cards as last resort.
-life cards are good to bypass the few and usable Nighmare DR cards - Nistarok is the real threat here and Artus becomes marginal.
The downside of rainbow deck is that you usually have to struggle against anything else than SoB clans and meeting SoA clans is instant boot to head.
Only Ghumbo will give you trouble, because he has high attack manipulation and good power and damage. DR is simple solution and then just beat the **** out of rest of the NM. Also Ghumbo is liability against good power reducers given enough pills.