If they have a robust random number generator it's pretty easy to develop a good resist system. The problem is getting someone or some group of people with authority to define some numbers to resist levels.
Assuming an unresisted spell lands for 1000 dmg, what should be the expected damage at resist 100, 200, 300, etc? Once some consensus is reached on that question it's simple to define an appropriate regression to map out the rest of the possible resist values. Then you use the expected resistance at X resist as the mean for random number generation when a spell is cast on someone with X resist.
The mechanics are relatively easy, the bolded part is what needs to be answered first.
You can generalize this to debuffs or groups of debuffs or multi-effect spells too, you'd just need different resist tables for each group.
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