I saw one MUD with a good solution. There were plenty of ways to get resistances, including gear with innate resistances, gear with temporary enchantments, and buffs. But the buffs wore off quickly, the temporary enchantments took a long time to cast (per piece of gear!) and wore off at boot (at the end of the day), and most importantly, resists were capped at 70%. It's that simple. Combine a cap in resistances with multiple damage types coming at you at once (an avenging angel might have a fire/holy/physical sword, plus lightning or wind spells, to name just one example,) and the problem of too-high-resistance-too-easy is solved.

That may not be the only way, or even the ideal way, but it's one way that worked well.

Mason