it's safe to say Tsunderplane's bombs are more AoE potent than Toriel and Asgore Fire Magic, also that Fire Magic are very small same size of Flowey and the Armors, Sword and Shield covers and deflect all 7-C soul damage and physically weak in physical level
unless they touch the body instead of armor pratically null in pre-battle, 50m is also easy since Alex/Steve have better mobility than Goats.
And Ghasts' fireballs can be deflected because they're just shooting Fire Charges at you, while they do not. Even if they COULD, they'd fly back where they came, not towards Asgore and Toriel.
Fire Magic is the same as Fire charge projectiles unless the projectiles same as Blaze's AoE fire (Also Fire's in Minecraft are constant and hotter than Nether Heat the Heat can evaporize water instantly surviving the Building Level AP), and it's possible to deflect the projectiles towards Asgore or Toriel due experiences.
Also
unless they touch the body instead of armor it's not problem if their souls need to cover or not (it's just spiritual level) and you're more leaning to Undertale's interaction
For adding Invulnerability frames into this topic for Minecraft there's
Immunity given 0.5s to any source of damage, even you don't agree, it only work when against Sans not Asgore or Toriel also since Frisk and Alex/Steve both classified as humans the INV interaction must be same as default otherwise verse mechanism would be contradicted.
I'm more agree with @Keeweed's argument. I late to notice this was Speed Unequalized meaning this battle thread is meaningless than Undyne vs Steve. It's also safe to say Vexes would kill Asgore and Toriel (being higher tier doesn't mean you're stronger than haxxed of lower tiers) in Speed Equalized. But Asgore and Toriel have good soul AP but Alex and Steve have also good physical AP which mean both side are strong nothing more nothing less.