Try to get mesmerize talent from tomes for the last levels. Mesmerize the golems or ranged enemies (spiders to be exact) and make them kill for you while you are safe behind. Another useful spell would be confuse, try to get the confuse spell from tomes, charm or wand. Confuse enemies make them attack each other while you clear the rest. Try to do one of the optional quest to get more items/upgrades/levels because no matter how strong your basic attack or main spell is there is a big chance that you can get killed in the last 3 levels.
Won for the first time as Fire Mage after too many tries. The last 3 levels are definately hell. Used wand of confusion when too many enemies suddenly appears. It works like a charm (see what i did) confuses enemies to attack each other. My advice is to keep your scroll of fear, domination, teleportation/blink(for retreat) for the last levels. To kill Crystal golem as a fire mage I waited near corners then use fireball so that it cannot be reflected.
So luckly I found a greater fire staff(+2) in shop(barely collect enough gold to buy it), enchant it 2 times (~16 dmg per shot), have enough mana to wreck havoc. Then your rng dungeon creation make my char enter the dungeon around 5 enemies, one enemy is happened to be reflecting my dmg by 100%. Could you give us a little chance to beat the game once in while?
You can use PunyMortal's walktrough for hard badge with storm mage too. Stand in one way corridors when enemies approaching you to use lightning bolt on multiple enemies. After you get "Storm Attunement", "Energy Conservation" and "Mental Fortitude" talents in that order, get "Static Discharge" talent for good AOE damage. Train focus and electric magic equally until both 8, then only train electric magic until max (20), then spend points on fortitude just like PunyMortal's fire magic built. Don't forget to use health and magic mushrooms before engaging enemy.
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Gonna send you a quick fix - hold on tight!