Approach6-7hrs

From the Grimsel hospice, follow the track, which starts by crossing the dam, to the end of the lake.  From here, follow the path markings through the loose scree to the Unteraargletscher. Go along this on its true left bank, passing the Lauteraar hut on the right, or follow the markings on the moraine, less agreeable, (loose rubble.)
Go up the Finsteraargletscher and go right where it meets the Stahlegggletscher, to reach, at last, the Aar bivouac.

Route5-6hrs

From the bivouac, Go up to the top of the Strahlegggletscher to the S gully of the Lauteraarhorn. Climb the long gully (600m-900m, as the access to the gully is already steep.)
From the col, follow the ridge on good rock to the summit.  (There is a pinnacle which is awkward to down climb, can be avoided on the left.)

Descent

By the same route