The Impossible trial 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 third impossible challenge you have to enemies inside the gold rings. Each rings gives you 4 minutes to defeat 10 enemies inside it.
The Ring respawns 2 more times in the arena, so you have to complete a total of 3 gold rings with 10 enemies each.
Once again Atreus is your most important weapon to complete this impossible trial smoothly. Use your chaos blades to dragin enemies inside the gold rings (L2+R1) and shoot all of Atreus’ arrows at the target. This way you can get enemies inside the circles and are able to almost instantly kill them.