The aforementioned solution is good but I want to point out one that worked for me in just a few minutes...
Leave your Palico in camp and bring a Ghillie Mantle.
In the Elder Recess, look on the backs of Gastodons. There are several locations for Gastodons in the Recess, but they really like the lava in area 11. They are also in area 5.
Always have your Ghillie mantle on when approaching Gastodons. Be sure to crouch if you actually see the Bristle, as Gastodons can still get spooked even with your Ghillie Mantle.