To unlock the Not Dead Yet trophy in Mortal Kombat 11, you will need to perform the Mercy move 10 times.
You can only perform a Mercy move after you have defeated an opponent and it says Finish Him/Her. Performing the Mercy move will revive your opponent with a small bit of health.
To perform the Mercy move, move to mid-distance (four steps away) and press
+
X 3 in quick succession.
Repeat this 10 times to unlock the trophy. If you want to get it done quickly, set the rounds to 1 and fight against a second controller.