Even if Magus is powerful, Schala still has techniques to own him. Firstly, she can't die because of her pendant.
Yeah, I suppose she couldn't die permanently... but she would die again and again and again then, because the fact that she has "auto-life" doesn't change the fact that she's less powerful than Magus.
Secondly, we know that she can errect barriers on things, so she could just trap him in a cage.
Then why didn't she cast a barrier on Dalton in Alghetty, when Dalton kidnapped her?
Thirdly, Magus lost energy to Lavos.
And the fact that Janus only hides a magical power that surpasses Schala doesn't mean that he surpasses her in every aspect.
Yes, I was saying "Janus is more powerful than Schala", not Janus surpasses her in every aspect (healing, effect magic, time/dimensional travel, etc.).
He probably has far greater shadow magic than her, and that is why later we realise that he could time travel and fly. Schala, on the other hand through CC is shown to be much stronger. If one second of contact with a Lavos that is draining your energy makes you loose like half of it (Magus), then what about 13,000 years for Schala? Even while in contact with him, she managed to create Kid.
This point might be the less trivial of this topic actually. Lavos wasn't absorbing energy, he was
merging with her AND she was merging with him. When she created Kid, Schala wasn't particularly weaker in magic, she just had her mind corrupted and broken because that's what happen when 2 different conscious beings merge together into "something else".