Simply, let them get 100%. Just also have mobs and items that can lower that value.

As an example, if you have a "Fire Shield Spell" that gives the player 100% fire resistance, then have a lava monster that casts "Fire Vuln" that makes the player 50% more susceptible to fire attacks. Thus now the player is only 50% shielded. Also, cap the resistance at 100% and take the Vuln's from the total resistance and not the sum of the parts .

Now an engagement looks more painful.