This is more of a query than an imformative post..
IIRC, when Zeromus uses Big Bang, it comes with a Spell Multiplier of 10, so technically it can't be evaded, but is there something coded to the spell or caster that makes it work 100% of the time? (The minimum spell multiplier success is always at 1.)
The same goes for Bio/Virus, since I don't recall it ever being evaded, even when the Spell Multiplier for that is somewhere like 8 or so (but always split), but should normally be evadable...
Edit: I may have overlooked the obviousness of the spell, but the only connection between the two spells is HP leak, which "could" be the the culprit.
Bio can be normally be evaded (used by the Ghost Knight/IceSoldier) and I'll have to double check if that's evadable by the FF4ET Eurkaryote/Prokaryote...
2nd Edit: There's also the fact that HP Leak is inflicted when Mind Blast (the Mage/Mind Flayer or Breath monster) is used...
We have EVEN MORE themes now, but some old ones are gone...