Re: Powerleveling and Vanish/Doom
In the SNES version, every boss can be Vanish/Doomed, except for the final bosses. I think there's a handful that can't be Vanished (and therefore, can't be instant-killed in that way), like WrexSoul, but there aren't many.
If you're at the end of the WoB (those levels sounds about right), I guess the Floating Continent. Ideally, I'd suggest just not wasting time leveling at all yet, but if you want to, Floating Continent.
Later on there are much better places to level up if you want, and you'll have much better Espers for better stat boosts.