As it stands, I'd say both sides have arguments that could give them the win. Regulus’ ability has the potential to end the fight almost instantly once he starts attacking unless Tatsumaki uses decoys. However, she would first need to witness Regulus' attacks before even deciding to use decoys, so I think it's unlikely she'd open with this strategy.
Tatsumaki's main win condition seems to be BFR. She could throw Regulus far enough, likely into space (despite statements of Regulus being able to come back from it), where he wouldn't be able to use Lion's Heart due to the range limitation with his wives (at least 100km), at which point he probably dies from the cold and radiation. This is something Tatsumaki could fairly easily do, but it's not clear if that's her opening move at the moment.
It has been argued that Tatsumaki wouldn't necessarily use her telekinesis to throw Regulus farther into space than necessary, at which point Regulus could come back. This isn't necessarily wrong, and in the case this happens, Tatsumaki probably wouldn't try throwing Regulus into space again since she has no idea how Regulus' ability operates aside from it making him a distortion in space.
Tatsumaki could however use her Telekinesis to move Regulus away at a distance that exceeds the 100km range of Little King (while still on earth), and then this gives her a chance to kill Regulus while his ability isn't activated. The counterargument is that Regulus could use his FTL speed and authority to return (though it’s unclear how, since he can’t react to his own speed). If he does, he may take the fight more seriously and resort to spamming AoE attacks, which would be deadly for Tatsumaki.
Since this message is getting rather long, I'll summarize what I've read so far (apologies if I'm missing anything).
TL;DR
Tatsumaki's advantages are as follows:
- Can potentially BFR Regulus into space or move him beyond Little King's range, making him vulnerable.
- Can use decoys that Regulus wouldn’t be able to distinguish.
- Can tell Regulus is an anomaly in space, which would prompt her to be cautious.
- Faster than Regulus.
- Most likely to act first since Regulus generally just yaps a lot.
Regulus' advantages are as follows:
- Invulnerable to all of Tatsumaki's attacks, but can be moved by TK. He could potentially ignore Tatsumaki's TK with his authority, but given the speed gap, he likely wouldn’t do so fast enough.
- All of his attacks are instant-kills, and his large AoE attacks could nullify Tatsumaki’s chances of dodging.
- His attacks are unexpected and FTL. Tatsumaki can’t react to them unless she’s already in motion before he attacks, since he doesn’t require gestures.
- If thrown into space, there’s a possibility he could return. If he does, Tatsumaki is unlikely to try sending him to space again (she cannot tell what his ability does, and has no reason to assume it has a range limit. If she throws him in space and he comes back, she would probably just assume this tactic wouldn't work).
There are arguments suggesting Tatsumaki could figure out Regulus’ link to his wives, but this scenario is highly unlikely and requires too many steps. Her other win conditions seem far more viable. Imo, this matchup depends on many "ifs" and speculations about how the characters would behave (how fast does Regulus attack, would he take Tatsumaki seriously, how far would she BFR Regulus, would he come back from it, etc.).