Below is a rant, you can skip it if you like up to the Bold part.

Lot of people have replied and based on that I can draw some conclusions. And I should probably create an accurate scenario of my current position and thoughts. So first of all, thank you to those who have replied. You have given me some insights. But lets start with some scenario thinking based on some arguments.

Lets say, hypothetically, that there is an early, mid & end game in Kamihime; I dont know where this ends nor begins.
Next up, assume that there are different tiers of Kamihime and they can be categorized as T1 through T5 ((T1 = Best - T5 = Worst) there probably should be more tiers, but this is for the sake of the argument). I think that creating a Tier list in such a way would be quite cool as it could be a collaborative effort by players!

For example a lot of people mentioned Amaterasu, I believe that we can all agree that Amaterasu holds extreme value Early through Mid and can easily be T1 or T2. Doesn't matter which she is for the sake of this argument. But that later on, her added value decreases compared to other Kamihime; that said she still holds value Im not discrediting that! She would probably Late be 1 or 2 Tiers lower than Early & Mid. I don't know exactly how much but I believe diminishing returns are the case. I believe this is due to two factors. First of all the healing cap and secondly debuf resistant bosses. Which some bosses are but not all! Let's move on with this in mind.

Personally I believe Im in the midgame.
Total Power: 35439 (Sol and Ryo-Oh are subs (Lvl 60), Fire Mammon isn't maxed yet, Svarog isnt awakened, Achilles equipped (300 SP from Heracles))
Weapon Grid: ATK: 15146, HP: 1070 (2 non fire weapons but they have the highest ATK/HP values, 4 SR fire weapons 3 of which are they raid spears)
Eidolon Grid: ATK: 6833, HP: 2757

Roster: Svarog, Hephaestus, Fire Mammon & Mars.

(as I was typing this I noticed dreamlitz mention assault value, no clue what that is D: If you can elaborate I'd appreciate it)

Im gonna go for Fire since I just adore the aesthetics and the waifu's. But when thats complete I'll focus on Light!

Now onto my question! Sorry it took a while since I expend a lot on stuff but I think it's important.

Given that my account is in the Midgame, thats my opinion at least, and that two scenario's can exist, non-debuff resistant bosses and debuff resistant bosses. Who should I pick for my MT. Ideally I'd like to create a 6-7 Kamihime roster and rotate them based on my needs.

As far as I'm aware Frey won't be in the MT after this one? Confirm/Deny?

Non-debuff resistant roster: Svarog, Mars, Vahagn/Frey, ?
Debuff resistant roster: Svarog, Fire Mammon, Vahagn, Frey.

Who should fill the ''?'' spot, personally I think Amaterasu as usually my Kamihime have 5-6k HP and her heal can still be valuable.

Some explanations below.

Now I think that the debuff resistant bosses exist, and might exist more in the future but thats for future MT; not the current one yet. Furthermore I can pick up Vahagn and Frey in future MT (which I plan on) and swap out Amaterasu and Mars.

Also I'd happily not take Uriel, if Frey is to some extent a substitute. When I came back from Uni I at one point considered muting the game whenever it was Uriel's turn so I could use her. But I'm really glad Frey can be a substitute, if thats accurate.

Special thanks to: Slashley, dreamlitz, KitKat and he who shan't be named the unregistered guest! (For their helpfull insights, this is really appreciated even though you guys disagree)