As we've stated, Doomsday has only one weakness – entropy. That is one environment he can never adapt to and if you let him die in such conditions, he will die permanently.

How is Doomsday defeated?

Superman finally ended the threat by flying the monster high into the atmosphere, and then slamming him into the ground, impacting with the force of a bomb. The blast killed both Superman and Doomsday.

Does Kryptonite kill Doomsday?

Doomsday possessed seemingly inexhaustible stamina, and was not noticeably slowed down from fighting Superman for an entire day. ... Once he gained some of Superman's Kryptonian DNA, he became susceptible to kryptonite. Unlike Superman, it caused him pain, but could not kill him.