First of all I would change leliana's weapon from a one handed sword to 2 dagger's or a bow. That way she can either spring an attack from distance. From what I can see with your main is that's it's a mage, no? Well use Winter's grasp. Has a chance to freeze. Then use I think Fireball. Very powerfull at range. Can remember but there is another fire spell in which you wave your hands rounds and it creates a vortex of fire. Most of all try and get Alistair to lure them away and always heal him if he is low or just make sure you have enough Mana and health Pots. Not much of a guide but honestly. It's done me no harm. Just gotta play around with tactic and see whats right for you.