Somewhat lengthy question here. Bear with me

My board:
Ob Nixilis, Unshackled (untapped)
Cemetery Reaper (tapped)
Big Game Hunter (tapped)
Thousand-Year Elixir (untapped)
Endless Ranks of the Dead
Thornbite Staff
and
18 2/2 black zombie tokens, 12 untapped, 6 tapped
8 swamps, all tapped
I have the staff equipped to one of the zombie tokens

My opponent casts Disaster Radius revealing Molten Primordial to do 7 dmg to each creature I control.

Do I have the option of using the zombie with the staff to ping away at his creatures as I lose zombies one by one, or is that damage from Disaster Radius applied simultaneously to all of my creatures. Same issue: can I kill zombies one by one to make Ob grow fat enough to survive, or does he die along with everything else?

Thanks! Gatherer is useless on this one.

The damage is dealt simultaneously. The untap ability from the staff and Ob's ability will trigger a bunch of times, but their sources will be dead before they resolve, so there will be nothing to untap or put counters on.

All the damage is dealt at the same time, so once Disaster Radius has finished resolving all your creatures are going to die at once--there's no way to have each of them die one by one and have triggers resolve in between each death.

Thanks! I figured as much, but wanted to double check.