[RPG] Are there any methods to destroy Brown Mold besides cold damage

The simplified versions of character creation are:

1. player uses a pre-generated character—either from "out there" or that you've built,

2. someone builds the player's character for him or her, or

3. player builds side-by-side with experienced person.

Your players have ruled out number 1, but I don't think there's anything disingenuous about presenting both options 2 & 3 as "player creation" options. It's just a matter of who's holding the pencil, doing all the little maths, and making decisions when the player doesn't yet have a preference.

During the last few months I've had seven or eight opportunities—some in my own campaign, some in Adventurers' League—to walk new and new-ish players through character creation. With some I've just had the discussion, absent any rulebooks, of what the possibilities are and what they might like to play. With others it's been sitting and flipping PHB pages with them, line-iteming the character sheet. In the first instance I'd estimate about a five-minute conversation followed by 10min. busywork; the second instance I find it likely to run a good half-hour. (It did on Wednesday, even with a guy who'd been playing for a few months and I was helping make his second character.)

Remember, the list of decisions to make isn't just race/class/background: it's:

1. Name (people find this one of the hardest)

2. Race, perhaps sub-race

3. Class

4. Ability score disbursement (go back and find those racial modifiers, too)

5. Choose some skill proficiencies, languages

6. Starting equipment: weapon, armor, perhaps another weapon, which pack to take, sometimes a fifth bullet-point which is mostly flavor and thus is second to "Name" in how much time it'll take

7. Archetype (Domain/Oath/School/Tradition/Circle/etc.)

8. Background—and now I've picked up different skill proficiencies and perhaps a language slot, so perhaps need to change original choices. I've also picked up different equipment, so perhaps need to change those original choices.

9. Let's fill in the weapon attack table

10. Magic: If you've got a spellcaster, "choose four from this list of twenty--they're all in the following eighty pages."

And there are many more pair-interactions than just the Background ones I explicitly pointed out!

So I'd say you should talk to these players about at what level of detail they want to be involved in character creation. If it's acceptable to have a five minute conversation with each, you spend a day filling out some character sheets, then another five-minute conversation discussing how you implemented their intent that'll speed things along a lot. You go away with notes on their interests, send them away with the basic rules pdf. Those what come back knowing all the rules—encourage them to re-tool their characters on their own time. And be ready to make small in-game tweaks to characters in recognition of the fact that it was you that wrote up the character rather than the player.

Specifically, the effects of Gunpowder exploding is covered in DMG 267, under Explosives. And Oil is covered in PHB 152, but only for a flask.

Broadly, rules for Improvising Damage is in DMG 249, where it lists various examples and the amount of damage, by increments of d10s.

If you don't like memorizing all those bits, or are still having trouble improvising the damage, you can read: The Angry GM’s Marvelous Mechanical Miscellany for Ad Hoc Adjudication and Improvisational Invention (WARNING! TheAngryGM link, contains swearing and beating people with their copies of the DMG). It gives clear guidelines on how to improvise setting the DC, and how much damage an effect should do. I'll quote some of it here, in case the link goes dead:

First of all, pick a baseline damage: d6 for a low damage effect, d8 for medium damage, and d10 for high damage. If the damage can happen more than once, reduce the size of the die. So, for a continuous effect or a trap that goes off every round, the damage is d4, d6, or d8.

Now decide if the damage is leveled. The idea is the same as the one I discussed above for setting DCs. Tripping and falling into a campfire isn’t leveled. It’s just stupid. Stout, hardy heroes are more likely to shrug off that blunder. So that damage isn’t leveled. But if the fire trap is in Tyracticus’ Fortress, you can damn well bet he’s using magical super hellfire. It’s leveled as hell.

If the damage is leveled, roll half as many dice as the PC has levels, with a minimum of 1. At first level, if you want to, you can reduce the die size. Up to you. I find it doesn’t make much difference and it’s only for one level. If the damage isn’t leveled, roll 2 dice.

I've personally tried this method, and have found that it is not as punishing as I like, so I stepped up the damage die in the baseline damage to d8, d10, and d12.