It depends what classes I have in my pt, but I will usually try to focus on killing the Darkspawn as my main priority so they aren't beating me up while I fight the AD. Also, having a someone(preferably a rogue, incase repairs are needed) at the Ballistas can be handy, not so much for the dmg aspect, but because it can be used to briefly distract the AD when it's trying to use a move that could potentially cause a large amount of damage to a pt member. Army assistance can be useful in distracting the Darkspawn your pt isn't currently engaged with so they don't gang up on you, and what I've found to work pretty well for me is using an army with opposing fighting style to my pt. For example- if the pt consists of melee hitters then I would use a ranged attack based army, and if the pt has effective ranged attack skills then I would use a melee based army. Also, potions are your friends!
Be sure to check your gear before going into the final fight to make sure you are wearing the most appropriate gear for the fight. The AD is pretty resistant to most elements, and while still resistant, it has a lower resist percentage to nature dmg. The AD uses spirit dmg dealing attacks, so you'll want your pt to be resistant to spirit dmg if possible, which you can simply use a spirit balm to achieve.
I hope my opinions are helpful to you without being too redundant from the other stuff posted. There is more than one way to win, you just have to find out which way works best for you.