Duncan And King Cailain's Swords
Duncan's sword, Oathkeeper, can be obtained in Lothering after the battle at Ostagar. Go to the chantry board and do the Bandits, bandits everywhere job. Then go back and do the two remaining jobs. The chanter will give you Oathkeeper. It has a strength of 8. 40 and can be enchanted once.
Cailain's sword, Yusaris, can be obtained at the Circle of Magi tower. You'll need to unlock the basement door. When you get to the floor before you meet the sloth demon, there will be a room with three statues. Touch the first statue, then the third, then go back to the second. Then later on the same floor, when you get to the room with the tranquils around a big statue. One of them is the final statue you need to touch to unlock the basement. If you do all this right, words above your head say that there is a rumbling on the main floor. After you beat Uldred (or you can just go and get the sword), try to go into the basement. A demon should appear behind you and attack. Defeat it and you'll get Yusaris.
Damaging The Archdemon
When you fight the arch-demon in the last battle you will soon find that it has a massive amount of health. To get good damage to the demon call in whatever allies you have and use them as blocks for your mage or tank and let them die in favor of your own followers. Also use the ballistas - they will deal from 50 to 120 damage on average with every bolt. Use the advanced key and have your followers hold positions by two of the ballista and quickly switch between them to pin the arch-demon down. You will get a moderate amount of health taken down in a good amount of time. It is a good idea to have a rogue with you in case the ballista jam but on casual you do not have to worry about it. Use cold spelly on your mages and cold weapons on your tank and warrior.