I found those 4 devourers in that little house in Undvik harder than Eredin lol
Try the following (i did it when i was drunk and wanted to find out how important skills are):
1) Deathmarch difficulty
2) erase your skills
3) enter endbattle.
You might be surprised how very possible it is. It just takes time... and getting used to the fact that you can't trigger certain skills that you are used to.