If you encounter Sartharion with 3 drakes alive, you can send one of your priests into the portal when Tenebron opens it. Priest goes in there, waits 1-2 seconds and uses Divine Hymn spell and get out of the portal when it is done. That causes to eggs disoriented thus there won't be any adds, all you need to do is killing Tenebron after that. If your DPS is low, you can repeat the process!
ps: alles nur geklaut :D
ps: alles nur geklaut :D