Since nobody here has offered any actual help I guess I'll take a whack at it.
First of all, this fight is one fuckfest of extra adds that you have to find a way to deal with. What you shouldn't do is try to kill them all. There's no point in it- when you're at the ballistae stage, they will never quit spawning until you have drawn the AD off of his platform. You need to keep Alistair on them, make sure he is in full defencive mode. That means shield wall is up, Wynne is focusing him, and if the horde becomes too much to handle, keep Oghren there to help trim the numbers so they don't overwhelm Alistair. Kite the shit out of them as you make your way between the ballistae. DO NOT LET WYNNE FOLLOW YOU. If she gets too far from Alistair and he dies, its game over.
Speaking of Wynne, you're going to need to whip her tactics for this fight. She should NEVER have any spell in her tactics that is not a heal. Try to be frugal about it, too- she should be using her direct heal at around 30-40% life, and party heal should be turned off, but keep it in her button map so you can dictate when you need it, don't leave this up to AI. Let her use a lyrium pot if she is at <10% mana, no more no less. If you did things right, Wynne will be autoattacking and conserving her mana for the duration of the fight, and will only be healing if Alistair or anyone else drops below 40%. Her damage potential should be relatively low, anyhow, since she SHOULD have higher willpower than magic stat, so you're really not missing out on any damage if you keep her this way.
For the most part what you want to do is keep Alistair close enough to you that you can help chip away at the adds as you hit the ballistae, which means you will need to switch between followers frequently to force them to move. Be very aware of what is going on in the fight. Sometimes a small group of darkspawn might appear and instantly aggro to Wynne, and in this case you may need to take control of Alistair to quickly intervene before she is dispatched. Alistair and Wynne are your two most important followers here, and if you lose either of them the fight will very quickly go to shit.
Once the ballistae stage is over, focus on the group of darkspawn you have on Ali and rout them as quickly as possible before refocusing the AD. Very few should spawn at this point, so you can resume the tank and spank on the boss. Hopefully this will help. I wish you luck!