Questions about fire ball with multi targets

[sasigns]

If you are using a fire ball against say 5 targets one of them gets advantage on his saving throws vs magic how would you apply that effect to just that one enemy ?

[Zacchaeus]

There isn't currently a way to create an effect which allows advantage against magic. The only way to do it would be to do that particular NPC separately. So the PC targets all of the NPCs apart from the one who gets the advantage then casts the spell and then does damage. Then untarget everything, target the one who gets the save make sure the DM has the 'ADV' button pressed and then cast the spell and apply damage.

Another way would be to create an effect such as ADVSAV: {ability} where {ability} is the ability needed for the save (so dexterity for fireball) and apply that to the NPC. The NPC could then be included with the other targets as normal.