If you wish to make occasional fights where a single element is a key point to the whole thing (like fire in the red dragon lair), anticipate it and create a countereffect and/or assume that any sensible adventurer *will have* a resist potion by boosting the general difficulty of it.
This would mean that in my game the volcano is a special location is likely to give +5 damage to attacks with the fire keyword or reduce fire resistances by 50%, or something similar. In this case I think it is ok, since it is a rare occasion (a dragon fight) and it would be ruined with everyone having fire resist.
On those random situations where a few undead enemies get trashed by the party when they have necrotic resist, who cares? Occasionally letting the characters gear make encounters easy is a good thing. At least they feel like they are getting something out of it.
This would mean that in my game the volcano is a special location is likely to give +5 damage to attacks with the fire keyword or reduce fire resistances by 50%, or something similar. In this case I think it is ok, since it is a rare occasion (a dragon fight) and it would be ruined with everyone having fire resist.
On those random situations where a few undead enemies get trashed by the party when they have necrotic resist, who cares? Occasionally letting the characters gear make encounters easy is a good thing. At least they feel like they are getting something out of it.