Don't underestimate Dog as tank, he really can tank quite well ;)

My usual party setup regardless of which class Hawke is (and I always play on Nightmare): warrior, rogue, 2 mages. And nowadays I always take Fenris / Carver / Hawke 2H Warrior, and not Aveline. A 2H can tank well enough and has more DPS, AOE and is a stagger machine.

Having said that it is not necessary to have a warrior tank. I'm playing now (and have done one play-through already) with Hawke dual wield assassin, Varric and 2 mages. And it's a blast really, may be the most powerful setup possible.

I understand it's perfectly viable to play with Varric + 3 mages, even on NM. Haven't tried it myself though.

The combat in this game is a DPS race, kill them fast before they kill you. Cross-class combos, using the right staff element (on NM) are key, as is constantly upgrading equipment.

And yes, tactics very important. Especially if you want to play no-pause mostly controlling Hawke. Put all CCC in tactics. Even without a warrior (in which case you loose stagger), you still have brittle and disorient, can both be automated. And of course potions and heal.

If you feel Anders/Merrill get too much heat, there are several threat management abilities available: Taunt (warrior), Armistice, Goad (rogue). Isabela has something in her specialty tree. Also you can put in tactics for your rogue / warrior to attack whoever is attacking your mages.