Instructions
Press "T" to open the skill tree. Select the skills leading to "conviction" for a paladin or "lower resist" for a necromancer, then select and raise that skill so it is available for use. Earn skill points if necessary by killing creatures and leveling up.
Click the icon for the right mouse button attack in the lower-right corner of the screen next to the mana sphere.
Select the "Conviction" or "Lower Resist" skill to select it.
Right-click the mouse on the ground to cast the aura for a paladin or right-click on the desired enemy with your necromancer. The enemy is no longer immune to fire.