Multiattack. The mammoth makes two Gore attacks.
Gore. Melee Attack Roll: +10, reach 10 ft. Hit: 18 (2d10 + 7) Piercing damage. If the target is a Huge or smaller creature and the mammoth moved 20+ feet straight toward it immediately before the hit, the target has the Prone condition.
Trample. Dexterity Saving Throw: DC 18, one creature within 5 feet that has the Prone condition. Failure: 29 (4d10 + 7) Bludgeoning damage. Success: Half damage.