Okay tonight I'm going to take on the Crestwood dragon (the Hinterlands dragon's babies make her too hard for me). I know I have to bring Vivienne. Can anyone recommend the two other companions I should take? I am an archer FWIW.
If you can, use Spirit Damage, i.e. use Viv's upgraded Spirit Blade A LOT. Also, make sure you unequip Viv's lightning spells, cause it's a lightning dragon.
I'd suggest bringing Blackwall and Cassandra. If you have Cass's Spell Purge talent in the Templar tree, it will stop the lightning dragon's attack that creates a field of electricity on the ground.
If you have it already, start your Archer off by doing Full Draw for the first strike versus the Dragon, then Mark of the Rift it. Then, go nuts! I'll go ahead and warn you, it will take a lot of micromanaging.
Also, what level are you and what difficulty?