I assume it is because they have lightning immunity, and the character that he described is using primarily lightning

Now I feel like me and my friends are playing dnd wrong. Ending encounters in 3 turns is very rare for us. Usually they last like ~6 turns. Sometimes more, but 3 and less is extremely rare for us.

You sure shield master doesnt work on fireball? The AC bonus to DEX save doesnt apply, but taking no damage on a succesful save should still apply, no?

DMs, would you consider adding rogues/assasins as enemies against spellcaster who spam fireball? Or like a group of dwarfs with shield master? On one hand, it will encourage more variety in a future, on the other the spellcaster would feel bad that he cant do much in such a fight and would probably get killed quickly.

It doesnt feel right “punishing” a spellcaster who uses fireball as a solution to all his problems like that, right?

Edit: and make them tieflings as well…oh god…poor fireballers