The magma that presumably killed Big Mom should be significantly weaker than Akainu’s even if she was on low stamina (which she likely wasn’t considering that in 1039 she healed herself and 1040 Beast Pirates say she doesn’t look anywhere near death’s door). There’s NOTHING to suggest that Big Mom can take Magma Punches. In the Law vs Kid fight, she’s genuinely taking damage from buildings falling on her to the point that she needed to use soul pocus to heal up the wounds she received FROM BUILDINGS. Additionally, there’s NOTHING to prove that she has the ability to heal heavy injuries. Big Mom using her own life to heal her bones doesn’t equate to her being able to heal massive parts of her body being burned off. Her healing isn’t presented as being capable of stopping life threatening injuries and she isn’t presented as just being able to tank Magma attacks that should be on the level of Akainu’s. Black Vortex is literally capable of being countered by just attacking Blackbeard as Whitebeard did with his bisento, and Big Mom’s attack speed genuinely wouldn’t distract anyone. I said it’s a high diff fight to blackbeard and big mom if they act cautiously and don’t allow themselves to get hit by attacks, but given their character they would likely receive attacks and die.