Elemental Mastery

When you gain this feat, you can only choose a same damage type you have chosen with Elemental Adept. You gain resistance to the chosen damage type.

Spells of 1st level or higher that you cast treat Immunity to damage of the chosen type as resistance(This resistance is not overcome by Elemental Adept). In addition, when you roll damage for a spell you cast that deals that damage, you can use dice of 1 level higher:
1d4->1d6->1d8->1d10->1d12->2d6