Look at the screen shot with death log. Feenja has 40K hp, then she's defeated during the second round after the boss uses his ultimate for the first time. I attached my lineup as well.
#In Grave of Rot Feenja just dies out of nowhere
outer topaz
unreal swallow
In S4 boss has this passive that slays heroes that have too much corrosion on them. I guess you used Theo as tank, and since he can't get shield this means all damage boss does to him on ulti is sent to all your team as corrosion.