The elemental protection perk (when you cover) must give 50% protection from cold. Having an additional 20% potion must give 70%. During my (lvl26, health 300) encounter with a wisp mother, I do not see ANY reduction in damage, covered by shield or not. Currently, shield protection against any types of projectiles does not work. Arrows and ice spikes hit you through the shield. Whether they shoot you in the knee, I don't know.

The insanity of ice mages in this game, against whom nothing works made me quit playing last time.

Have you tried a ward of some kind? They vary in strength though. Since you're level 26 right now, you might opt to go for the better ward spells. They can be bought at the College of Winterhold.

PS: If you think the Wisp Mothers and Ice Mages are bad, wait until you meet the Arch Cryomancer....