The 28-year-old Hirscher, second to Kristoffersen in the overall World Cup standings going into Friday's race, carried a 0.23 seconds advantage after the first run.
Racing last in the second run he looked to have gone off course when he slipped on the icy snow but recovered with a superb move to finish just ahead of Aerni's time.
The Austrian also won the giant slalom at Alta Badia last week and is firmly on course for a record-extending seventh successive overall World Cup title despite suffering a broken ankle in a training run in August.
Despite his dominance of the World Cup, Hirscher has never won an Olympic gold medal - something he has a chance to change in Pyeongchang in February.
The Pyeongchang Games run from Feb. 9-25.
(Reporting by Karolos Grohmann, editing by Pritha Sarkar)
