D&D 5E Tips for Resolving Area Effects

[Saeviomagy]

For the largest groups, I'll just look at the save number they need. That percentage makes the save while the remaining percentage fails. If they need a 13+ to save, that means 40% save while 60% don't. If it's within a few points I'll usually just round it to 50%, 66% or 75% and move on. I usually just have the targets closest to the epicenter or in direct line of the effect fail and those on the fringes succeed.

This is way better and more interesting than group saves. It prevents every monster in a group from falling at exactly the same time just because they were all taking the same AOE damage, meaning your casters have to deliberate on whether it's worth casting a fireball when 60% of a group have already fallen.