If your Altaria has a high attack stat, use the Life Orb. If not, use the Choice Specs on your Altaria (only one move will be locked when you give a Choice Specs to your Altaria until you switch it out into a different Pokemon). This powers up your special attack stat.
Since Sandslash has a low defence (Unless EV trained for defence), use Focus Sash. If you want your Sandslash to be a physical sweeper, use a Life Orb.
Hope I helped.
