Cultists definitely aren't a guaranteed run ender, although they're certainly a tricky puzzle to solve. When playing, try looking for spells that would specifically help you beat cultists. Area of effect damage spells, long ranged damage spells, silence effects, spells that give you actions, or spells that deal damage without having to cast a spell at all (row of strength or row of sacrifice, for example (there are others as well!)) are all good ways to handle cultists.
Hope this helps!