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 challenge.
For the first impossible trial you have to defeat 20 enemies without taking damage. The best way to approach this is to max out all damage, chain and rapid fire perks from Atreus. His ranged bow attacks are you main weapon in this fight and.
Also get the Traveller’s chest armor which can absorb one hit every now and then. So you can basically fail once during this trial and still complete it.
Once it comes down to the last 8 enemies an ancient will appear and attack you with his ranged attacks. As soon as you are at the last 8 enemies, use all your special attacks and take out as many enemies as quickly as possible. It’s very hard if you have to juggle the ancient and 7 other enemies so keep the number as low as you can at the end and focus on the ancient to complete the first impossible trial.