for me I am playing as a sword&board warrior, I travel most of the time with Cassandra as a S&B as well, Varric or Sera as archers and Solus full specialty and barrier and then some into ice.
I have the extra potion slot upgrade and on Cassandra I use the Pitch Grenade, my character the Antivan Fire, on Varric/Sera Jar of Bees, and a Confusion Grenade on Solus.
Try to have your focus max'ed so the special ability can be used. I stack them at the start of the fight by using the tactical overlay, Pull of the Void, Rally, Rain of Fire, and my archers' one. After those are use open the tactical overlay again and use all the grenades except for the Confusion.
At your level I would say most of the dragons would be down to 50-60% health in a matter of seconds just doing that.
I do not focus on or even really care about the fire/ice/lightning resistance on my guys. I keep my ranged away from the dragon out of reach of their melee and breath attacks, while keeping barrier up on my melee by using on cooldown. It would be rare if your team barely even hits 70% health like this.
I know some will say two S&B is ridiculous on the dragons that summon adds my character picks them up while Cassandra stays on the dragon. Also, if you are crafting your own runes and weapons you can make 1 handed weapons that are close to the damage of 2-handed ones. for example the 1H axe I make with rune is sitting at 344 damage while the 2H axe is 382. Yes there is a 38 damage difference but the shields I am using added another 44 armor so the tanks are like walking fortresses.