Make Hawke your tank with 2H Reaver/Vanguard skills mostly. I dont spec for taunt or defense skills except for elemental aegis.
Get Varric, Merrill and Anders. If you get melee companion, Hawke's will oneshot him with friendly fire. Varric is CC/DPS, Merril is CC/primal DPS and Anders is Support healer. With this setup, nightmare becomes so easy because the party damage output is so huge that mobs usually die within seconds. There is also plenty of CC. eg. crushing prison, petrify, miasmic flask etc. I keep up Heroic Aura, Elemental weapon party buffs and set up tactics to manipulate cross class combos.
I nightmare positioning is very important because of friendly fire. I always keep the range away from hawke, and I try to take now mages or rogues ASAP, or at least CC them.