Thinset would be a much better alternative to an organic mastic. I recommend application with a 1/4" deep x 1/4" wide square notch trowel.
It is advisable to properly tape your durarock joints with fiberglass tape and thinset before the tile is installed.
You can probably still use the mastic, however, it will not be as strong as thinset mortar. Thinset mortar greatly adds to the sturdiness of the wall you are tiling.
If you are going to use mastic, skim coat the wall you are tiling with the mastic the apply your mastic over the skim coating normally and then tile your wall. That way you will recieve the most out of your mastic bond.