Much like Flare-Marie, the first 9 waves are tests of your roster and CE diversity, so you may need to do a lot of prep work. Have the Gamepress page on this fight open on one screen and the Chaldea app on another so you know what to kill and how. Focus on the enemies that drop more medals.
Most importantly, keep pausing and resetting the waves until the result is acceptable to you. You don't need to kill everything. If you can't deal with a wave, focus on surviving it. Use the auto setup feature to quickly setup the next waves while you workshop and refine the one you're trying to beat.
And whatever you do, DON'T WITHDRAW. You'll lose all progress. Pause, farm, level up skills, try again.
Wave 1: Do you have AoE foreigners? Grab Idol X from the leaves shop and get her levelled if you haven't. Support her with other AoE servants levelled and prepped to fire NP's round start. Focus on survival and killing the bigger tentacles for 3 turns.
Wave 2: Do you have AoE Avengers? If not, AoE berserkers are probably your best bet. Otherwise, survive. (Maybe save Kiyohime for wave 8.)
Wave 3: A massive pain in the rear. Caenis heals for 100k per round. 1* Bunyan's 3rd skill reduces that healing to 70k at rank 1, and to 50k at rank 10. Bring your strongest ST saber, and support with extra divine damage if possible.
Wave 4: Do you have an AoE Arts Lancer? Arts looping will be easier when hitting six enemies at once, especially with the right supports. You may need NP damage to kill the tankier ones.
Wave 5: Another massive pain in the rear. Do you have Ignore Def CE's? If yes, you'll lose out on CE NP gain. Neither of the two Ignore Def archers have good looping potential either.
Wave 6: Aztec-city-name-here but a Ruler, not pretender. Also has perma-invuln. Do you have an Avenger that you didn't use in wave 2? GIve them an ignore invuln CE. Otherwise, use a ST beserker. (A braver man than me would try Salome.)
Wave 7: Can you loop with casters? Keep in mind that they come in three at a time over five turns, so spread out your skill timings. Avicebron does good damage, and there's always Chen Gong.
Wave 8: Do you have Taisui Xingjun? All of the dragons have unremovable guts, and Barty has six. Curse, poison and burn are your friends here, as they'll kill the 1hp dragons after guts triggers. Like with wave 7, pace your AoE and skills.
Wave 9: The biggest pain in the rear besides the boss. Do you have an MLB black grail? Stack as much NP damage you can onto your ST riders and use the first two turns to build NP gauge. Dagon is 'Threat to Humanity', Divine, Demonic and Super Giant. If you can't fill those niches, focus on surviving.
Turn 37 - Wave 10: Aqua-Marie
By this point, I can only provide guiding principles to help you take her down. I intentionally left Castoria available for this fight, while you may not even have her. You know your roster better than I do.
- The max amount of medals you can farm is 230. If you can get that many, you probably never needed this guide. Save those medals for the 50 cost invuln and the 5 cost NP gain/skill recharge.
- DON'T USE QUICK CARDS. Attacking her with quick will give her a 20k heal per round, per quick attack.
- Attacking with arts will give her def, which can be outlasted. Attacking with buster will decrease your NP gain, which can be circumvented. But attacking with quick will set you backwards.
- Bring debuff clear. She will routinely apply 'NP seal' and 'negate healing', which is a death sentence on the wrong turn.
- I highly recommend Asclepius. Even at 1/1/1, he's invaluable. Teamwide debuff clear + NP gain, 3k heal, and regen, guts and debuff immune on NP.
- According to her traits, she's Human. Apparently.
- (Izo viable?)
- Don't be scared to bring a saber, assassin, or one of the other classes not listed in the help tip. No disadvantages is an advantage in itself, provided they can stay alive.
- (Izo? Viable??)
- After her first break bar, she's guaranteed to max her gauge, empty your own, increase skill cooldowns by 2 and AoE Extra attack. Time the break to line up with your survival abilities or use 50 medals to cover yourself.
- All the other affects from her break bars can be outlasted. For this reason, I recommend a slow and steady approach.
- Be mindful of her final break bar, which removes buffs from you. Buff removal will take the newest buffs first, so try to trigger survival skills before less important buffs
If this didn't help, or I'm flat out wrong, let me know so I can edit accordingly.
Have fun, and enjoy the final convo!