Pre-Help at the Final boss Tiamat

Tiamat is dragon/divine so Martha is very effective as third skill affects divine enemy.

Merlin didn't die he come back as support for 2nd last battle until fou kick him out of the party.

Both permanent buff also active for the last fight. It best if you can bring debuff remover for last battle. Tiamat have several annoying debuff such as 3 turn NP seal, skill seal, buff block, atk down, def down. A team of Martha, waver and support Medea Lily is best for last battle. NPC support for last battle is Gilgamesh Archer so just ignore him. My front line for that battle is Martha, waver and Nightingale. Backline Mash, Nobu (also effective against divine enemy) and support Medea lily. My front line only Nightingale die.

Altria Pendragon (Archer) can solo the fight pretty much. Just take her with you and should be easy.

I frontlined all my damager with the guest Gilgamesh Archer instead of borrowing a waiver(I cant help it. His pretty cool at that moment lol). And I did fine. I think you'll be fine too. I just Just focus hitting with st nps.

My scàthach (lancer) did about half her health using her NP and 2nd and 3rd abilities (anti divine and quick cards more effective) in the same chain immediately after an Ozy NP (defense down). Honestly I think scathach alone could carry you as I completed it in very few turns.
You're Martha ruler should hit hard and 3rd skill has no reason not to work but if you pair her and Scáthach together, there's a grand total of 2 arts cards so perhaps bring A CE or two which guarantee NP gauge increase per turn? One of Tiamat's attacks does hit gently and a lot to increase your NP gauge for you too

Overreacting a bit. Don't forget the blessings of the underworld goddess that gives you +10k hp increase along with attack up and such. Spamming st np will take care of her if you don't have anti divine servants.

Well u are a bit overreacted. In that battle, u already have Martha which means u have good dps to deal Tiamat. Just be advised to bring team defense-buff servants (like our best Kousai, Hans, Waver) to sustain and utilize Tiamat's damage as charge for u NP. it is not hard than most other boss fight in that chapter.

1st battle, use NPC Merlin
2nd battle, use any good support like Hans, Mashu etc, or NPC Gil if u have good critical team for him.