Gugalanna, the sumerian Bull God, was slain by Gilgamesh and Enkidu. Gilgamesh is described as a demigod, but Enkidu is fully mortal.
In general, there are few circumstances where a clearly-defined non-god has killed a clearly defined god, but much of that is because deific status is never as well defined in mythology as it is in D&D. Real-life figures from greco-roman and egyptian history have laid claim to godhood. On the opposite side of the spectrum, celtic mythology survives only through christian adaptaions, and the celtic dieties are described as mere historical figures.
What leads to the "deities can be slain by mortals" reasoning isn't so much mythological precident of actual deicide, but the mythological descriptions of gods and their powers. Compare the abilities evidenced by a 30th-level Wizard to the abilities evidenced by the norse god Loki, and your Wizard ends up kicking ass. Or consider the story of divine exploits that begins the Volsunga saga: The dwarves Hreidmar, Fafnir, and Regin are enough of a threat to Odin, Loki, and Thor that they can compel the gods to pay a weregild.