With your current hp level, your best bet is to go in during BT with FB then hope for the best. If you join a light ult before it's at half health, running Joan as soul actually allows you to survive normal OD if you have additional atk down even at that hp level (I used to run Joan, Tish and Sol with ~7k health and can nibble it slowly down to half health before my party completely falls apart from the death beams.)
The best way I find to deal with light ult is to hold your FB when it's close to half health, and FB as soon as it rages. Re-send support requests at that point and just hope a lot of ppl join and everyone just throw a FB at it (or pray that a vet joins - if you're in an active union, see if you can coordinate with them.)
If you have Berith, Joan Ex Saint Breath or Andromeda Ex Fortitude, or Atum auto revive (there might be others, too, but those are the ones I know of), those all help you survive another death beam. But you can't really turtle it out after it rages, so just bring your best spike dmg build.
Another strategy if you know multiple players with Amon U and/or Archangel, you can actually try to stun the light ult. You want to use all the mode gauge reduction skills first then FB AFTER that - if you FB before, you waste a lot of the reduction 'cos it's a % of remaining bar. Once stunned, it's not gonna rage again before it dies, and non-raging light ult actually isn't all that difficult.
For dark ult, it's another one that you can't realistically turtle out unless your hp is very very high (the DoT all have caps, so very high health helps a lot.) Best strategy for your current power level again is to FB during BT and hope for the best.
After FB, either try to die as quickly as possible so that the boss regen doesn't erase all your dmg, or the safer option is to bring Naberius (dark R from gem gacha, you will draw her eventually) and use her zombie to turn the boss regen into a DoT. This is raid-wide, so it really helps when you have a bunch of new players who don't know they're healing the boss just by taking turns and doing puny dmg.
If you're going the Naberius route, your goal is to stall as long as possible and let the regen DoT nibble down the boss, so bring as much heal and cleanse as you can get your hands on (EDIT: make sure you have a way to cleanse the zombie off yourself before you start healing!!!) I would use Mordred Ex her orb eat skill if you have it since that is the best way to stall. Also, do NOT use any buffs on the first turn - dark ult casts overheated on you during the first turn, which is considered a buff and therefore cannot be cleansed. However, one of her move also cleanses your buffs, and will actually cleanse the overheated if you don't have other buffs in front of it.
At your hp, you can't realistically survive the OD with all the DoT on top though, so you just kinda have to throw bodies at it if no vets join your raid.
Others may have other tips, but those are the ones I know of for your power level. Hope that helps.
EDIT:
There's a lot of RNG involved with the charm and blind - it's possible that it never procs, in which case you can keep whacking at it, or it procs 100% and you do no dmg AND you're not building up burst. Having self burst gen like Shingen, Aether and Arianrod in your case probably helped a lot.
I thought those come out like 8 months or so from now? And to get T4 souls, you need a maxed out soul weapon AND 150 regalia of every type, kinda a catch 22 if you can't beat all the ults first...Quote:
or wait for t4 soul that has 60% cut with no relic weapon needed