Sooo easy! I tamed one BEFORE I unlocked the saddle! Just get a ballista burret, load it with trang arrows, and shoot a bronto! It will attack you, so hide behind a appropriate rock or cliff and shoot again, and again, and again! When it runs from you, continue shooting. It will knock out and feed it mejoberries!
Brontos are a bullet sponging machine if found with correct base health and melee you will make raiding easier can get 150 Brontos to around 70k health and swiping it's tail can destroy x seeds hella quick