If the player has a build that's at least decent then:
Mage - the Bane of Loghain. Seriously, this isn't even funny, magic counters him so hard and he can't resist all the spells in your arsenal. He will be mostly frozen, petrified, paralized, horrored, hexed, cursed, drained, stuck in crushing prison etc for most of the fight. The exception is a defense/support mage focused on Creation and anti-magic. Not many play this build but if that's the case just have a companion fight instead.
*Arcane warrior - activate your sustains, right click auto-attack and watch Loghain fail.
Dual Wield Rogue - you should have high Dexterity thus high defense score so some of Loghain's attacks will not connect at all. You also have 2 good stuns in your kit (Dirty Fighting and Riposte) but it gets even better: bring Concentrated Deathroot Extract. He will be stunned for most of the fight.
2 Handed warrior - activate Indomitable and win.
Sword & Shield Warrior - activate Shield Wall and win
Dual Wield Warrior - you are OP as hell. Stop complaining.
Archer - this late in the game you should be auto-attacking like a boss with good crit chance. Pinning Shot to immobilize and Shattering Shot for armor debuff are all you need plus auto-attacks, just AA him to death. He will knock you down a lot but it should not be a problem, you have high defense score because of high Dexterity.