Spawned adds have a strange timer. First attempt they didn't spawn until well a minute in. Second attempt I had two spawn within a few seconds. Then they came at odd intervals with the final round just being 1 spawn.
Spawned adds are not mezzable hower it looks like they hit for 3k max maybe. I found it easiest to just burn this down and leave the adds up slowed. The "Skeletal Instability" was survivable with spell shield buffs and some runes - but maybe I just got lucky not getting hit while my health was low.