best spell selection to defeat Loghain?

Tough one, better you had to stick with rogue or warrior instead, but a mage can use winter's grasp and poison cloud at best as a combo to fight Loghain. Just stack on enough poulteces of both health and mana, for just in case.

Mage = use all your control spells available and keep on the move. It's quite easy to defeat Logain without ever being hit once.

Cone of Cold, Blood Wound, Winter's Grasp, Crushing Prison and tons more. Earthquake, Petrify, anything that stops his movement. Just keep him locked down and keep your distance.

If spells are resisted or on cooldown just keep moving. Melee in this game is screwed up and targets must be semi-still for them to be hit, as long as you are moving away from him he can't hit you very often.

the best way is to impede his movement and lower his attack.
I'd say, use in this order: cone of cold, paralyze, winter's grasp, lightning, misdirection hex and repeat once they recharge, you might need to take a few potions between the time misdirection hex ends and you're able to use paralyze again (petrification might not work on him, so I didn't use it)

blood wound and crushing prison its very hard for him to resist for damage you can go for arcane warrior build if not shapeshifter bear form also can do heavy damage it depends on what your specialization is
a) Blood Mage- Blood wound (imba spell)
b)Shapeshifter- Bear form after movement impeding spells (pertrify , winter's grasp , cone of cold , crushing prison)
c)Arcane Warrior - use crushing prison and start whacking him after that hit with mind blast if he resists just run around dont get too close
d)Sprit Healer - Cant help u much in offense but good survival ability
But basic spells needed are stunners like mind blast, paralysis or mass paralysis etc followed by damage spells like rockfist or drain life etc