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.
