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.