I was playing the last round of Sim and it was the 5* Doc Ock mission. I attacked Phoenix and downed her. Enemies turn and she popped back up from the ashes. Her power is a 1 turn countdown, so she shouldn’t have risen until the following turn. I believe I was using Doc/Gamora/Medusa… I don’t think anybody on the enemy team had any powers that accelerated countdowns…
There are a few safe ways to do this… if every red tile is already a special tile and no new ones drop in, or if 5* Thanos purple is out it will prevent a countdown from being made, but generally, it’s best to save her for last even if you think you have a pretty good way to destroy the countdown.