I finally did it with the vaettirs but I'll try next week on a reroll.
The wiki says to flag back the SoS Rt. I think I should put it to bodyblock with other heroes. His spirits can BB and it might avoid the desaggro.
What did happen : my heroes killed the group of foe too slowly and then a new group arrives. Sometimes, the heroes can kill everything before an other groupe of foe but sometimes they can't. At this point, it becomes very hard to handle everything before the next foes.
Thank you for you answer anyway
