Regrowth: On units, this spell will regrow the unit from at least one model. The first replaced model is always multi-wound (Krox in a skink unit), then the champion, then the rank and file. If the unit is gone, you are out of luck.
On characters, if the character dies, that is too late. You cannot bring them back.
Dwellers below will kill any model that fails the strength test, no saves allowed. However, I believe you get a look out sir roll.