The Impossible Challenges in God of War unlock in the Realm of Fire after completing all 6 trials. When you return to Muspelheim, the trials are back and unlock keys now. The trials have also changed and are now called impossible challenges.
For the second impossible challenge you have to defeat all enemies in 4 minutes.
Compared to the first challenges where you had to kill 20 enemies without taking damage, this one is a lot easier. Make sure your level is at least 5 and you know your special moves. This impossible trial doesn’t really have any suprises when it comes to big enemies.
Simply deal enough damage and you should have no problem getting through the second impossible trial and collecting trial key 2.