Killing an argopelter

How on earth do i kill one?
It wont stun when i stomp or bullrush it. And when i try to pull it away so i can slash it abit, it just disappears, because there is no trees. But if it's close to trees it just climbs up and it's gone.
It wont stun when i stomp or bullrush it. And when i try to pull it away so i can slash it abit, it just disappears, because there is no trees. But if it's close to trees it just climbs up and it's gone.