I have played all 3 types of characters and I can assure you that the Mage is by far the most powerful character in the game.
I was suprized when I knew that I had to fight Damiel
I think you mean Damian and not Samuel right? Well Damian took my Mage about 2 minutes to kill using only 14 medium mana porions and 2 small health potions. I was doing massive damage to him with every spell.
If you are having trouble with configuring your Mage you should ask specific questions and they will be answerd to help you out. List your stats and skills when asking the question so that we have an idea what your character looks like.