Sandstorm is a pretty vague term. Some think of it as a team where every member takes advantage of the Sandstorm weather effect in some way, usually by being resistant to it. I personally would define Sandstorm as any team that capitalizes on the weather effect, while not being entirely dependent on it. For example, a team utilizing Tyranitar to set up sandstorm and LO Aerodactyl to gain the SDef boost, but including enough support members to fix up weaknesses would work.