In the original, the party was fixed so using Rydia there was mandatory, while on Advance you could replace her with someone like Yang. Dying or not, she's useless anyway. Edge is kind of not very useful either, while Kain is cool mostly for jump.
Tbh it's hard to remember lvs exactly but i'm pretty sure it's doable without grind since i'm mostly too lazy to do it nowadays (Unless it's something you can do really fast like metal slime stuff on modern DQ) and you normally reach the final dungeon on like 40-43. Since Cecil have like the double of Rydia's HP, he should survive a big bang even being 15 levels lower, i think. Atleast Cecil, Rosa, Kain and Yang should survive a big bang on 45.
I know the snes american version was based the easy type mode so Rydia definitely survived, and i was sure the same happened in the GBA and PSP versions until your post.
The advance version is the one i play the most nowadays but being fair i replayed it 3 years ago so there's a chance i'm talking nonsense.
Zeromus wasn't an easy boss by any means but i think he's easier than that optional boss that gaves Cecil's ultimate weapon.
I'm one of those weird peeps who keeps the party consisting of members that make the most sense in the given context of the fight. Switching to Yang or Cid would've probably been better, but the problem wasn't the DPS. Withstanding Big Bang was the main issue throughout the fight, coupled with ensuring that Curaja is casted no matter what.