">Because of this, we can work out that the Hotlands is many times hotter than the surface of the sun."
@Super Ascended Sean Pazdera also mentioned this which explains how the fact did the spilt water & the styrofoam cup (including the water inside it) evaporated instantly.
So everyone who lives and can tolerate the heat in Hotland should have "High Resistance to Heat Manipulation" since surprisingly none of them complained about the 9000+ F heat, even the Royal Guards (except for RG 02 since if you clean his armor, you will weaken his resistance to the heat and also, if they have armor on like Undyne did in Hotland, then why did they never experience heat ex....welp, Toby's logic apparently -.-) but however,
only Dr Alphys cannot have this since she lives inside her lab like 24/7 and
a total stalker watches over Frisk in their adventures throughout the underground, the usual irrelevant things you already know.
Another thing, like what @Kepekley mentioned earlier, add their Building level durability feats to their VS profiles.
Here's the list of everyone I was referring to that can tolerate Hotland's heat besides Frisk: Sans, Mettaton, Vulkin, Tsunderplane, Pyrope, So Sorry, Royal Guards, Muffet.
Let's see what's next, since Pyrope has Heat Manipulation, he should be able to easily control the heat of Hotland (9000+ F) and increase even further with more Pyropes being summoned to help him out. So, therefore, he can scale to this.
Don't forget about Mettaton's Forcefield Creation:
https://imgur.com/SrGKP7Q
Lastly, I have a suggestion about this:
https://vsbattles.fandom.com/wiki/Undertale
May you replace the front cover of the Undertale characters with their Tarot cards? I was just asking about it since it looks like good thumbnail material for the fictional verse.