How to kill Uldred on Dragon Age: Origins?

The trick to the fight is to use the litany every time he's going to convert a mage into an abomination.

I did that and the fight went fine - I don't remember it being particularly difficult.

If I remember correctly, Wynne shouts out each time you need to use the litany, so if you've killed her, this could be a bit trickier, but I think both Uldred and the mage glow white when he's about to convert them

If you have a mage with Cone of Cold abuse it. That can freeze any enemy (even the Arch demon) Should at least give you time to beat on him for a few free seconds. Also As mentioned in Durathor's answer:

The trick to the fight is to use the litany every time he's going to convert a mage into an abomination.

Also if you have anyone that can use poisons, most of them do a lot of damage and don't cost you any Stamina or Magic to use. So use them.