This is to demonstrate how certain characters would fare against others from their universe.
Meridian
Higher level characters she could defeat
VS Winner
Night Storm is one of the most powerful members of Gaia Force, but it has already been shown that Meridian can take her Sunburst almost point-blank. Night Storm is also weaker than Meridian and is not as skilled a fighter, since Night Storm relies on distance fighting with energy attacks. Meridian can take all Night Storm can dish out. Night Storm could only survive if she hid within a star or turned intangible.
VS Winner
For the same reason that Meridian can win against Night Storm, so it is against Hartree. It is, however, very possible that Hartree could drain Meridian's guardian energies. If Q'Aestor can do it, she must be able to do it as well. How this would alter the fight is unknown, since Meridian's connection to endospace could be a factor.
VS Winner
Meridian and Geomass seem to be evenly match in terms of strength, but that is because Meridian as yet to unleash her total power. Meridian's strength is tied to endospace, which enables her to bypass certain laws of normal space. This gives her an almost unlimited strength. Geomass's meteorites would not help him against her.
VS Winner
Meridian and Roguestar have had their share of mock battles and although all of these were for sheer fun, Roguestar knows for certainty that Meridian would win against him in a battle to the death. His concussion blasts could slow her down a little, but they would never overwhelm her. As for his Nova Blast, she has taken it full force many times.
Lower level characters she may NOT defeat
VS Winner
Caregiver would not hold out very long against Meridian, her strength not being in her class. Caregiver's florakinesis would do little to slow down Meridian as well. But Meridian greatest weakness would be that she would need to fight hand to hand against someone who can pull out her soul, and so kill her instantly.
VS Winner
Meridian certainly has the power to defeat Renshi, but doing so is not a forgone conclusion. Renshi has fought her many times in training and knows all her weaknesses. His fighting style is quick and deadly, and he knows which nerve clusters to hit to incapacitate her.
VS Winner
It could prove futile to fight against Arkahnus since he could erect force-fields to defend against Meridian's attacks. She would eventually break one down, but then he would form another. Arkanus could also trap her in a force-field and teleport out of reach. He can also teleport people and has teleported Gatari inside walls, killing them instantly. Whether it could kill Meridian is still debatable.
VS Winner
There is no doubt that Meridian is faster and stronger than Spell Weaver, but Spell Weaver's magical arsenal could defend against her attacks. Spell Weaver also has the means to entrap her and restrain her, and could possibly whisk her away to another place or dimension.
VS Winner
Psylash would win for the simple reason that her telekinetic powers have been shown to be able to restrain Meridian. Once restrained, Psylash could use her empathic abilities to mess with Meridian's mind. It is also very possible that Meridian could eventually escape her restraint, but the initial victory would be Psylash's.