From Ex illis wiki
Similar to Toughness, this value shows the resistance to element-based damage. High Resistances reduce the enemy debuffs’ chances to work. For spells using more than one element (Poison Wind) or all elemental values (prismatic spells), the average of all related Elemental Resistances is worked out and applied. Elemental Resistances are reduced by Veil.
These values increase with Levelling using this formula:
Final Resistance = Basic Resistance X 1.029183Level (gains 33% every 10 levels)