Ruling Weakness vs Resistance?


Poet lost in static.
I am not sure if this situation has ever come up before but I was wondering which has first priority when deciding damage out put of an attack weakness or resistance? Because this recent set we have golurk

Because of its ability, it hits dark Pokemon for weakness and resistance at the same time so for example if it hits m tyranitar EX

If weakness is prioritized over resistance the attack would do 240 and then minus 20 meaning you deal 220 damage
If resistance is prioritized over weakness then you do 120 minus 20 doubled so you end up dealing 200 damage
It's even worse when you add jolteon attacking a m rayquaza EX because it could mean the difference between a one hit knock out and missing it by 10 based on which gets prioritized weakness or resistance.