It has some ridiculous damage reduction my 1100 damage rifle was doing 80 damage but I believe it was because I was shooting its back.

If it cannot reach you it will run away for example like me I was on my tapjara trying to kill it for levels using guns so I was out of its reach and it would run away from me then I would get low to the ground and it will start chasing me again.

To kill it you need a tapejara with around 2k health (for trial and error), faster than it, flies, a lot of stamina (optional), a lot of damage, and plenty of patients.

To kill you take your tapejara and start riding it to your fiend (titanosaur) and start attacking its neck, head, or tail, after a second or two I will either run away or come after you if it comes after you it means you are at the elevation where it can hit you.