Take the turnoff to head left down to Biberwier. Shortly before Biberwier, follow the ever steeper, wide road towards Marienbergalm mountain hut. Once you reach the Sunalm mountain hut, follow a short but very steep path up to Marienbergjoch ridge (the first part is possible by bike, later you will have to push it).
After this, it is downhill all the way. From the Marienbergalm down to Arzkasten and back to Obsteig.