Elemental Visionary

When you choose this feat, select a damage type from fire, cold, acid, lightning, or poison. You must have already taken Elemental Adept for that damage type. 

Whenever you deal damage of the chosen type, that damage treats immunity as resistance.

In addition, when you roll damage for a spell you cast that deals damage of that type, you may reroll any number of the damage dice. You must use the new rolls.