【FGO】The 4th U-Olga Marie Boss Fight & Guide - E-Stellar Marie「 Ｅ－ステラマリー 」

You should know the deal by now.
Beast Class Score will be unlocked after you beat her, in case you're wondering.

The usual team restriction for Olgamarie Boss Fight applies (one-time used only for both Servants and CE)
Must make 10 teams of 3 Servants
You have access to a special "Order Change" in this battle, where you can instantly change the current team to the next one in line. This skill has 1 turn cooldown. Once you get to the 10th team, you cycle back to the 1st team.
The fight has a timer of 25 turns. If it reaches 0, it's Game Over. There is only one way to stall the timer, but only if you manage to reach Stellar Marie on Wave 10.

Now onto the fight itself:

Wave 1: All Servants' HP reduced to 1
Just use Aoe Casters
Wave 2: High Damage Reduction in exchange for decreased Instant Death Resist
Either use Instant Kill or a strong ST Servant if you want to brute force
Wave 3: If you do not use Order Change on the turn it appears, all Servants will get massive Atk and Def down, lose 5k HP, then Cerberus will gain massive Atk up, Def up and self charge to max
If you're still using the Caster team, it's time to swap (they're gonna die to the HP drain anw)
Wave 4: Left enemy resists everything except Quick, middle enemy resists everything except Crit, right enemy resists everything except NP dmg
Use AoE Quick NP followed up by crit
Wave 5: Class Triangle reversed (this was troll af for a blind run)
Use ST Archer Servants
Wave 6: 3 stacks of Max HP that expire after 1/2/3 turns
Either wait out the max HP buff or brute-force if you're strong enough.
Wave 7: Decrease your NP dmg depending on how many Offensive buff you have and decrease your IK resist depending on how many Defensive buff you have
You need 7 Offensive buffs and 3 Defensive buffs to nullify its gimmick. Hyperbuff your strongest ST Rider and hopefully oneshot it
Wave 8: 10 enemies with a limit of 5 onscreen at a time, continuous reinforcement
Due to how continuous reinforcement works, you can wipe them all in 1 turn if you use 2 AoE. I recommend an AoE Berserker followed by an AoE Moon Cancer
Wave 9: Self-buff its Atk, Def and Max HP for 1 turn every time you use a buff.
Try to buff up before you go into this wave, I guess
Wave 10: Stellar Marie

4 HP bars total
Shielder has offensive advantage
Saber, Ruler, Foreigner, Beast have offensive advantage and defensive disadvantage
Archer, Assassin, Pretender have offensive disadvantage and defensive advantage

Every turn, she gains a stack Quick/Art/Buster "debuff". Hitting her with the correct card type will reduce her resistance to that card for the turn and, more importantly, freeze the timer for that turn. This is how you stall the fight until you can defeat her
Whenever she kills a Servant, she gains Atk up, NP dmg up and NP charge
She also gains a stack of Resist down when you have at least 50 stars on field

First break:
Increase her Max HP for 5 turns and increase her Atk/Def for 3 turns at the end of every turn for 5 turns
Second break:
Remove all buffs from your Servants, then inflict skill seal/curse/def down/np drain every turn to them for 3 turns (some of these cannot be cleansed)
Third break:
Inflict Fear to your Servants for 3 turns, then increase her own HP for 5 turns and remove all debuff on herself
Gain a buff where she will fully charge her NP gauge every turn unless there are at least 50 stars at the end of the turn
The requirement to apply Resist down to her increased to 99 stars

How to deal with this fight, generally:
Mashu is your best nuker for the final HP bar if you lack a good nuker of your own.
Get to Wave 10 as fast as possible
Whittle her down while avoid breaking the 2nd bar with Mashu team unless you prepared countermeasure (buff removal resist and debuff immunity)
Save Mashu for final bar, preferably boosted by Merlin/Oberon

Timestamps:
0:28 Wave 1~9
18:50 Wave 10: E-Stellar Marie

#fgo