Oh, I know, lets compare Nuada to Rama (the premade Saber)!
now, Rama has the super-high-level divinity bonus, but you're talking about reworking that, so I'm going to strip him of it for this sim.
and since I think Nuada is OP, I'll give Rama the first turn, to see if stacking the deck slightly in Rama's favor changes anything.
Now then, Rama uses his NP on turn 1, to give him maximum firepower. spends 50 mana (dropping to 30), and automatically deals 60 damage to Nuada. Rama's PS blessing of martial arts activates, adding another 15 damage, dealing a total of 75 damage, automatically, on turn 1. (if he had his divinity, that would be an automatic 100 damage, holy crap, um, maybe this premade is broken too? GM!!)
Nuada gets to respond, and starts off with 25 mana, his passive activates, giving him +10, so he has 35 mana, I would have him use his NP, but he doesn't have enough mana to do that. wow, okay, welp, Nuada swings anyway, agi vs agi to hit. b + 4 vs a, so nuada has to roll (equal to or) under a 12 on a 10 sided die to succeed and rama has to roll (equal to or) under a 10 on a 10 sided die to dodge? what? I think they both get automatic successes? is that right? that can't be right. I'm going to pretend it's right. so..... what. I'm going to pretend it hits and just ignore the rest because what.
so Nuada hits, roll for damage, strength b vs endurance b. lets say Rama and Nuada both get 1 success because fk math atm, I'm hungry. So Nuada does 40 damage base, +20 from his passive, cut in half by rama making 1 end roll. so nuada does 30 damage to Rama, Rama is down to 0 mana. Rama's passive that is identical to Battle Continuation triggers. Rama is now at 10 mana.
Rama's turn. Rama's passive that is identical to Nuada's triggers. Rama now is at 20 mana. Nuada is at 35 mana, and I'm hungry. fk this, fk math, hits, damage = (50+15)/2 = 37.5. Nuada at 0, Battle continuation triggers. Nuada at 10.
Nuada, now at 20, identical turn to last turn. Rama's dead.
Both of those characters are stupid, rocket tag is bad, Nuada > Rama OR Noble Phantasms kill yourself. I think it's the latter and that both are OP, I'm leaving for food. brb.
edit: am eating now, thinking better again. I think that Nuada is perfectly reasonable when compared to the sample character Rama. I think that because mana is health, using spells and Noble Phantasms is problematic due to self-damage.
also, if we play out Nuada vs Irine, i think it goes like this: Irine uses NP, Nuada takes 10 damage, has to make opposed Str rolls, B vs E, Nuada wins easy. Nuada heals 10 damage (now at 100) Uses NP (now at 50), hits, no crit, irine gets 1 end success, Nuada does 50 damage. Irine is at exactly zero and doesn't have battle continuation. I THINK you need at least a little overkill to actually take someone down, so I'll keep Irine alive but it's iffy. Irine's turn, I'll be generous and give her full power passive +25 mana to put her at 25 mana. Nuada takes 20 damage (now at 30), and has to make opposed str rolls B with bonuses vs D, Nuada wins hands down. Nuada's turn, heals 10 (now at 40) and hits Irine again for the same damage as last turn. 25-50 = -25, Irine's dead, if divinity still added damage, Nuada would have killed her on turn 1 because of her own NP doing an automatic 50 damage to herself. Also, over the course of the fight, Nuada did more damage to himself by using his NP than Irine did over the entire thing.
regarding Nuada's NP. lets compare it to a burst damage NP for a second. if he had a straight-up Excalibur NP, it would deal 100 damage for 50 mana AND 1 turn's action.
with Nuada's NP, it does 40 damage / turn for 50 mana and ZERO turn's actions.
I actually don't have any problems with letting him keep it at full power, because he is paying the mana price for that kind of damage, and it takes 3 separate hits for it to outpace a straight-up excalibur. I think you should just make it take his entire turn to activate. time is a very valuable resource. case in point, look at Irine's NP. IF you pretend that it starts at A-rank power, it'd be stupidly OP. Why is it not a problem? because it costs a ton of time to get there. I think you can let BP keep the +40 damage for Nuada that he likes so much, just charge him a time cost for it.