How to kill a God Part 1

 This is mostly a stat workshop for a friend

As such I'm not gonna worry about the lore aspects beyond Fara is an imposter god of lightning. She was a mere titan who usurped the proper lightning god, kinda similar to Orcus. She was imprisoned for that shit and for some reason that's not working out so now the heroes are on their way to kill a would be god.

I feel like I learned a lot from trying to run a God Slaying Encounter of my own about a year ago, in which I did something pretty right but a lot of things really wrong. In the end though, my players felt like they had fought and vanquished a god, and that's what I was aiming for.

Okay hold up. Let's talk about killing a god.

How to Kill a God

Firstly you'll need weapons worthy of such a feat. Deities are immune to most mortal weapons and recovering the weapon or the material that can actually harm them should be a not insignificant aspect of the quest to kill a god. Spells certainly won't impact a Deity and as such she'll simply ignore spells below a certain level. Lastly, while a Gods physical form could die, unless you follow them to the Astral Sea or bind them in the material, it's usually impossible to actually slay the god with physical means.

D6 ways to kill a god -- 

1.  The Mud God draws his power from the earth and can only take damage if they are separated from the ground. (Grappling a mudgod is hard but possible, Gravity reversal would also work, etc) 
2. Ooblek the Ooze God can rearrange his organs at will, regenerating and restitching himself from even the smallest scrap. If Ooblek has less than 100HP at the start of the round he instead has 100HP. (Simple DPS check, stasis and time magic, haste, and action surge could all seal his demise, this information should be known before the fight) 
3. The Huntress Arduin can only be killed by an arrow which has been ritually prepared in the blood of a Stag in heat. (This should happen on screen. Make them track the stag, have it flee, let her divinely intervene in vague ways) 
4. The War God Achilates has never been bested in combat, but prophecy says his death will be at the hands of a trickster. (The final blow must be a sneak attack, this might not be obvious to the players from their initial investigations but if they look into what the prophecies means they should be made aware of this caveat) 
5. Apollon the Sun God is protected by his sister Navara the moon. If he is slain, or endangered, she will heal him. The only way to kill him is during the new moon, however, he is at his strongest during the day. (This one has a lot of nuance, make the decision to fight at night under the new moon impactful, Apollon knows he can only be killed during this time and will try to flee into the darkness and let his followers hold you off rather than fight) 
6. Hekate the Goddess of Magic has no true physical form and can only be killed by the legendary mage slaying sword Weave Reaver, which is attuned to the weave of magic and cuts spells and flesh alike. She is also immune to magic and magic weapons (Resistance to normal weapons, immunity to magic ones and she can only be dropped below 1HP by Weave Reaver, a sentient sword which can only be attuned to by a non-caster who has killed at least 10 mages with it) 

I like some of those, I don't like others, but it's 3AM so fight me.

The key here is that the actual mechanics of D&D combat are mostly what I would call "Solved Problems" There is a Most Effective Tactic to utilize a characters stat block, and most players know what that tactic is by their 3rd or 4th level as that character. Attempting to add wrinkles to this combat in the form of hazards traps and encounter diversity is great but a Deity should go beyond that. A barbarian could swing at the Mud Gods 14 AC forever, but he has no HP bar as long as he is connected to the Earth. Meanwhile Apollon knows about the narrow window to kill him and will try to flee into the darkness when challenged.

You should probably not fight the Mud God

None of these have explicit solutions grounded in the sheet there is no built in answer in a classes kit. (Except for Achilates but again, fight me) Suddenly your players are making decisions, and choices that aren't grounded in "What does my class do?" or "What is my best standard DPS option?" instead you'll have Barbarians trying to wrestle the Mud God off the ground (dificult and he will try to escape the grapple every round) or else find creative ways to push him off a cliff etc. The mage might haste the fighter so he can keep up with the Fleeing Apollon, but can the fighter and the monk 2v1 a God long enough for the main party to catch up? Or is it better to haste the ranger so that he can better track the fleeing God?