Having completed int once on I've tried playing this on the Viking difficulty, I decided to have another playthrough on God. The regular enemies I can handle, but now every time I touch the Soul Stone, I must first complete the Ether segment where Loki is chasing me in a labyrinth, and catches me every time. How can I progress if I can't even save my progress? Any tips?

After you complete the Ether the game will save

That's all well and good, but the Ether segment is too hard. Many times Loki spawns right in front of me as I'm carrying the energy ball. After a long and hard segment in any level, going to the Ether just to save and have a 50:50 chance (or worse) to succeed or play the tough segment all over again is very discouraging.

Well God difficulty is meant to be extremely hard, you are good if you can complete it.