Sometimes I'll go to a new area and have to fight 2-3 enemies at once, and usually I only have enough mana to kill one person. Then I have to retreat, potion up, etc.. As a pure mage am I doing something wrong? Almost every level up bonus stats go to Magic. Are the "guild" quests more difficult than the mainline story? Or does everything scale accordingly to your level? I'm in a dwemer ruin in a part of the Magic College quest line and everything in here takes ages to kill. With my melee character and my archer/rogue, there were times where they'd probably take a lot of damage, but it never took too long to kill anyone except for "boss" enemies.
Make sure you're using some good shouts in there with your magic, they can help out too. And what sort of mana regen buffs do you have on your gear? That can really help you last through multi-enemy fights
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