Well I don't disagree that mages are quite powerful but if you find them too powerful simply play another class... All classes can use all skills.
I'm finding the end game with my mage (late demon area) on hard to be some what difficult. Perhaps it is my technique. I found the late game (hass dragons) very easy with my warrior on hard.
You have 3 classes and 3 level of difficulties. If the game is too easy with one class at highest difficulty just switch to another class
btw not sure how you obtained 4000x7 damage. I'm seeing around 3500 but against fire resitant creatures (demon) it is much lower. My warrior on the other hand just went around casting soul drain. 10k to 20k every other turn at the start of combat is wicked. Phantom on large stacks is also pretty wicked esp since the ai likes to target the phantoms.