If you want to kill Malthael without always made stage 2/3 before the boss (last quest don't put in you directly in front of him), just leave fast after a kill, the save point appear approximatively ten seconds after the kill. Just leave (tp&quit) before he finish his animation of death and when return, you can take the portal and be in front of him directly.and rekill him.. etc... etc.
Just be fast to : Pick up loot, Teleport in town and quit.
Good to farm Reaper's Fear for Reaper's Wraps craft.