Oh i see it. Actually this is a good one battle.
If this is wm obito, he would use jinchurikis first.
Regulus hasn't any regeneration negations or sealings on his profile to deal with edo jinchurikis, so the battle would be like this:
First obito sends yagura, i dunno what's regulus' first move here but likely space manip, but yagura counters it with his mirror jutsu (as per canon, his first move is this) so his attack would be useless. After that obito uses utakata to create bubbles for killing regulus in several explosions. So regulus gains several damages or escape from there but gains normal damages. Now regulus uses his water danmaku. But yagura's water mirror is still active, so he counters it again. After countering, obito immediately uses fu to blind regulus temporary. At this moment regulus uses his precognition but due to being vague, he only can find that two shinobis are coming closer to him and they with their fast speed comes to attack him. Yes they are han and yugito. Yugito uses her claws and han uses his steam amped strength to crush him. Regulus gaurds it, but gains a very important damage. But he really doesn't care for damages because he has high willpower.
Now regulus flies, obito sends three jinchurikis to put him down: Fu, Yugito and Roshi. Roshi goes to LCM, Fu opens her wings, and yugito uses her long ranged attacks. Now fu flies to put him down. and yugito attacks him with her blue fire which is homing attack. I believe regulus makes some defences via matter manipulation, but suddenly roshi jumps high and breaks the defence. Now i think regulus wants to use time stop. But after getting hit by roshi and falling into earth, suddenly obito appears behind him and BFR or kills him.
Notes: regulus' attack invisibility won't work cuz not only obito himself has rinnegan and sharingan, but also his edo jinchurikis possess them (rinnegan can see invisible things as it's capable of perceiving invisible barriers, and can even see the Invisible Limbo Clones, which reside in a different dimension and are normally impossible to perceive and sharingan can perceive invisible individuals and entities)
Regulus fear manip won't work on obito because he resist it by scaling higher than kakashi who can resist zabuza's fear manip
Jinchurikis overwhelming attacks won't let regulus to amp his speed, so he won't do it in this battle
Obito doesn't use genjutsu at first because this is out of his character. Although he can use that in any times he want. His 3T genjutsu is layered which is above regulus resistance (in ms his genjutsu effects enhance much greater, also regulus hasn't resistance to Illusion and memory manip tho). But i don't think it's necessary in this battle. In my scenario, he didn't use it.
Shared vision counters regulus hard so every attack and movement of him would be recognized by jins and obito immediately. Also obito himself has sharingan which analysises regulus so great and copies the power that he used.
Sharingan can do this:
The Sharingan provides its user with an incredible clarity of perception. This allows the user to easily keep track of fast moving targets with astonishing clarity. This clarity of vision also allows the user to easily perceive and decipher hand seals at great speeds, read lips, and perceive pencil movements in order to mimic them. The Sharingan also allows its user to focus their vision and act like a microscope, allowing it to see objects on a cellular level and clearly perceive viruses. The Sharingan allows its user to clearly anticipate their opponent's next moves and flawlessly counter them. A fully matured Sharingan can even see a clear visual image of the opponent's next moves, allowing for efficient evasion and counterattacks without any wasted movements. The Sharingan has the ability to "discern the reality behind any technique" and then reflect it upon its caster. This enables the user to read the way a foe weaves seals and determine the change in chakra nature and the range of the attack from the color of chakra. The Sharingan's ability to acquire and decipher this information allows its user to instantly understand, memorize, and mimic the opponent's Ninjutsu, Genjutsu, and Taijutsu with nearly perfect accuracy. The Sharingan's ability to see chakra also allows its user to assess the opponent's remaining energy levels and their next moves.
So that's why obito makes this plan (which is even on consistent with canon). He finds what he gonna do so he attacks with roshi and puts him down which makes obito to do a BFR or a killing jutsu on him
Conclusion: Obito wins after a good fight.
Yagura won’t be able to reflect anything, Regulus’s attacks are FTL while Yagura is relativistic, that’s a 10x speed gap, the mirror requires him to perceive, form the water, chant the jutsu (atleast in the anime he does) and rotate it in time to reflect, he won’t even register the attack before it lands, at best, if he's far enough and gets lucky, he might reflect one, but the reflection turns into water on impact and can’t actually damage Regulus since he cant be damaged by his own attacks, so even if it hits, it does nothing,
Fū’s scale powder would just create a flash of light, it doesn’t affect the eyes directly, Regulus wouldn’t lose function of sight, he’d just see white, if it’s AoE, she’s risking blinding her own team, if it’s focused, she’s giving away her position, and Regulus only needs to danmaku the light source with FTL sand or water and she’s gone,
Han and Yugito physically attacking Regulus wouldn’t even scratch him, his body is under constant time stop via Lion’s Heart, he
literally cannot be damaged, he's not tanking their attacks, he's not experiencing them, no matter how strong or fast they are, their attacks can't interact with him, lava armor, claws, steam punches, none of that matters,
The “important damage” claim is false, he’s completely uninteractable (body/mind/soul), that’s the entire point of Lion’s Heart, his body doesn't register any change at all, so they could blow up the planet around him and he wouldn’t feel a thing, except for the fact that now his wives are dead and he's back to the 5 second limit and dies in space.
Rōshi’s lava chakra mode also means nothing, heat is change, Regulus is immune to change, he won’t burn, won’t get hot, nothing.
The genjutsu argument doesn’t hold, Regulus isn’t “resisting” genjutsu, illusions, or memory manip, he doesn’t need resistance because his mind and soul literally cannot be touched, his body, soul, and mind exist outside of the world’s flow, so nothing can interact with them, no sensory, mental, or soul based effect applies.
The Sharingan bit doesn’t matter, Predicting movements is pointless when your opponent is using FTL attacks and your body can’t interact with theirs in the first place, seeing what’s coming won’t change that you can’t block or stop it. And i dont think the sharingan can work effectively on something 10 times faster.
