2-Take the checkpoint
3-Kill Malthael
4-When Malthael is dead take all loots quickly , press escape and leave the game.
5-The quest "kill Malthael" take about 15 sec to be validated you must quit the game before. So if you doing this fast 1 sec to take all leg and 10 sec to leave the game.
just after that you can restart the game just at the Malthael portal again.
Enjoy loots