Loki

Vidar will stride forward, as Fenrir gloats over Odin's death, and thrust his foot into the wolf's lower jaw.
Vidar's two feet are different. One of them has a normal shoe on it. The other wears a shoe taht has been constructed since the dawn of time. It is assembled from all the bits of leather than people cut from the toes and the heels when they made shoes for themselves, and throw away.
(If you want to help the Aesir in the final battle, you should throw away your leather scraps. All thrown-out scraps and trimmings from shoes will become part of Vidar's shoe.)
This shoe will hold the great wolf's lower jaw down, so it cannot move. Then with one hand Vidar will reach up and grasp the wolf's upper jaw and rip its mouth apart. In this way Fenrir will die, and so Vidar will avenge his father.