Stomp Udam
IMPORTANT: You must have the Mammoth Rider skill from Karoosh's skill tree to do this mission.
Head over to Karoosh's map marker over in the cold northern area. If you haven't crafted some winter clothing, best do so now. I like to craft the first level of winter clothing for a bit of immersion since the final clothing gives you immunity to the cold. It is a personal preference but I think it is nice to look for warmth sometime. This is even remedied easily by setting your weapons on fire or to make it faster, burn a forest to make yourself warm.
Head over to the bone tree the Udam made to start the mission. They create these bone trees from mammoth bones.
Continue on to the camp of Udam. You will find them making fun of a mammoth before killing it to make their bone trees.
You can choose to kill all the surrounding enemies or just make a mad dash to the mammoth and ride it. The special controls are shown beside the mini map.
Destroy the Udam bone trees littered across the camp. They are those spiky looking structures you see around Udam camps. This seems to be their production facility.
After filling up the destruction meter by messing up their bone trees, go to the marked location and bring back the mammoth to its herd.
TIP: His other mammoth friends may or may not take too kindly to you especially if you have a predatory beast with you like the Sabretooth Tiger. So be prepared to run or kill the aggressive ones if their icons on the mini map turns red.