According to https://datacrystal.romhacking.net/wiki/EarthBound:Status_PSI_weakness_value the values of PSI Flash resistance among enemies is 0%, 10%, 50%, and 99%. However, in enemy_configuration_table.yml it is given as 5%, 40%, 70%, and 100%, like the values for PSI Fire and Freeze resistance. I did some testing, namely trying Flash on an enraged fire plug, which should either have a 0% or 5% flash resistance, and I wasn't able to get it to work once. It could be that datacrystal is wrong, I'm not 100% certain.
According to https://datacrystal.romhacking.net/wiki/EarthBound:Status_PSI_weakness_value the values of PSI Flash resistance among enemies is 0%, 10%, 50%, and 99%. However, in enemy_configuration_table.yml it is given as 5%, 40%, 70%, and 100%, like the values for PSI Fire and Freeze resistance. I did some testing, namely trying Flash on an enraged fire plug, which should either have a 0% or 5% flash resistance, and I wasn't able to get it to work once. It could be that datacrystal is wrong, I'm not 100% certain.