I've heard people can fight Loghain with their party against his, I was able to fight 1 on 1 with him. I fought him using my main Rogue/Duelist/Assassin. I don't remember I was at, maybe 18-19? But anyway, Loghain is susceptible with stuns so I kept using Dirty Fighting and Flurry to whittle him down, paused and queued up a health poultice when my hitpoints reached to about halfway. I kept following that process until he died. He was a pretty easy fight.