Get the Longneck Rifle and some simple rifle ammo and with your Argentavis or Pteranodon fly to the most tallest rock where he can’t damage you and your pet shot that mf and that’s the most easy way

Alpha Carnos will attack and kill anything it sees, so when your close to one, either run away, or attempt to kill it and get the Alpha Carno arm.

To get it:

Search for alpha carno

Use a therizino or spino or Rex (can use baryonyx if underwater)

Kill it:they aren't tameable unless u hack the system

Boom you got the trophy