Asterius and his accompanying berserking bisons can take out your entire party if you don't spot him coming. Be careful about aggroing him if there are monks near him as it can take a while to take the mob down.
In normal mode, it takes him 8-10 minutes to patrol his route. In Hard Mode, it will take 2 minutes.
If you are hunting him, go just outside Olafstead and get the bounty from Olgana Beartamer. Head immediately to the shrine that Inga Caveborn is standing near. You should be able to catch him coming up the hill near the shrine. In Hard Mode, head slightly westward from Olgana and wait there instead; his increased movement speed will bring him along in roughly a minute.