Ramuh uses his Blood Pacts on timer, every ~22 seconds, starting from when he uses his first Lightning Armor. 'Feeding TP' is not a thing.
Stunga does not cause him to go to the second person on hate list, but it does remove the main target's hate. Who he targets is random. He can use Stunga between his other Blood Pacts.
Bolt Strike can be blocked with 3 shadows.
Light skillchains healed Ramuh, Distortion did not. It could be any light-aligned SC
I resisted stun from Shock Spikes very frequently using Tellus runes, but it was not guaranteed.
You can kill the summoner before Zantetsuken goes off. Funny thing is Odin will still kill you from Zantetsuken anyways, but Odin will despawn and you will win the fight.
It looks like it takes 3 of them per round, but I was stunned from hitting him. I don't think I was getting stunned from Bolt Strike, so if you could disengage and still keep hate (maybe /BLM or /BLU) then you could probably do it.
NIN doesn't need to engage if you are tanking. Yonin and cycle shadows. The enmity generation from shadows alone is enough to keep his attention. Can't get stunned from spikes if you don't hit him. Sirvente if you need extra hate, but shouldn't.
Shiva spamming sleepga on Phase 2 is a real problem. We just got stuck in a Shiva Sleepga > Carbuncle Healing Ruby cycle that was inescapable. Eventually Fenrir stripped us all of enough buffs to kill us.
Elshimo Pachira Fruits or Poison potion and Shiva is a non-issue. Killing Leviathan and Carbuncle first if there is a number of avatars is better, since they will use frequent heals on the pack.
Fenrir can be slept with Repose on VD.
Lullaby also works, resisted on everything else. Probably makes sense to sleep Fenrir and kill the others, since a full buff would be problematic.