The Rise of Tiamat is an adventure that ends with the killing of the dragon queen who happens to be a god.
Now, they do not say kill Tiamat, but they do say banish (The Rise of Tiamat p. 88). Yet, from what I am reading in your question, it sounds like your god is currently living in the material plane and the bad guys want to destroy that corporeal manifestation, rather than destroy the god for good.
In that case, killing a god is the same as killing any monster, only way more difficult when directly fighting one on one with such. Actually, you are not likely to survive an upfront assault with a god, just like with an ancient dragon. Yet, a nice poisoned drink, as suggested, should work just fine! (assuming the god is not immune, so not an ex-Paladin...)
But the rule present there clearly says that when Tiamat reaches 0 hit points, it transfers back to Avernus.
Now, if you want to kill a god for good, as its essence go, you have to kill it on its plane of existence. Note that this is similar to killing a Dragon, Lich, or Mummy Lord in its lair...
The reference of killing a god can be found in MM p. 67:
If it [a Devil] dies outside the Nine Hells, a devil disappears in a cloud of sulfurous smoke or dissolves into a pool of ichor, instantly returning to its home layer, where it reforms at full strength. Devils that die in the Nine Hells are destroyed forever—a fate that even Asmodeus fears.
Asmodeus is considered to be a god (Deities & Demigods¹, p. 90) at the same level as is Tiamat.
An interesting addition from the Sword Coast Adventurer's Guide, p. 20, by the Sage of Shadowdale, in case a Revenge was sought:
"If the gods can grant the power to raise mortals from death, why do ye assume they should be laid low by it forever?"
¹ Reference to AD&D (1e) with lore that has not changed very much over the ages. From what I see in the Sword Coast Adventurer's Guide and MM 5e, this is most certainly not going to be different in 5e.