How do I get hidden ability tyrunt and hidden ability amaura?
Unfortunately, Hidden Ability Tyrunt and Amaura are not available yet. However, they might be available in future.
Hope I helped. :)
Source: Serebii's Pokedex Pages
The only ways to get hidden abilities for Gen 6 Pokemon (for now) are through Hordes and the Friend Safari.
Since Tyrunt and Amaura don't appear in any hordes or friend safaris, it is impossible to get Hidden Ability Tyrunt & Amaura.