I’m not going to lie to you this one will be tough. But the fact that you can say that you’ve managed to reach half HP taiga last boss standing tells me that it is likely possible.
One thing I found about this fight is that there is a good deal of RNG involved. You can’t get past that. All you can do is move it around. Command seals act as ways of controlling RNG. If you can guarantee an NP you don’t care about your NP gain. I was planning to save these for the end of the fight but I didn’t end up needing to use them. The plan was to have my Brave Liz work with as many buffs as I could manage and then just CS NP as much as it seemed necessary once I had broken Taiga’s bar because Brave Liz had a guaranteed two forms of survival with her built-in invincibility and with the guts CE - guaranteeing three nice NP turns, maybe even buster brave ones if she was alone when she did them.
What you want is some servant that can clean up Taiga at the end. An ST Arts or Buster saber. But that’s not all, really you also want to have a support for them. My leaning is towards trying a support Merlin and your Lancelot. I say this because there are no low rarity ST Sabers with hard invincibility you can bring here like I did with Brave Eli. I think you’re just going to have to pray to RNGesus and crit damage that it can close is out. Hero creation’s crit damage buff is insane, make sure you save it for after the break bar. If you can’t do that, the run might not be good enough. But regardless of that it needs to come out on a turn you can make the most use out of it. His invul will also give you a good turn if things go right. I honestly just think the support Merlin is going to do more for you than the support Musashi or Nero.
Be ready to use command seals. Don’t waste them on HP, that won’t guarantee you live only your hard survival will. Just find the run and execute her. Don’t be afraid to savescum to manipulate RNG. You can do this by popping a skill, or more easily by messing around with hitcounts.
The game has some kind of internal counter for RNG, and each additional hit count will move the counter forward to some other random value. So like say if you wanted Taiga to target someone else than she did on a given turn but didn’t have a taunter, you could exit out and choose some combinations of cards that overall have a different hitcount than the one that you ended the turn with and the RNG will potentially have a different result. If you mess with it enough chances are you can get what you want. Make sure to account for potential brave chains as well. If you didn’t have enough options from your hitcounts, you could just pop a skill that would be irrelevant or that wouldn’t be the end of the world if you used it a bit early - like the debuff cleanse on the Atlas mystic code since this fight doesn’t have much of that.
It’s not even limited to changing someone’s targeting, you can even affect your own ability to crit, the skills the opponent will use, the enemy’s ability to crit, and basically anything that is just RNG that you might be taking for granted.
For more information - 8min53s to 12min55s