A pure sweep team needs to be able to take a hit, yes even if they are sweepers.
For example a team like this:
Azelf (levitate for earthquake), Heatran (Flash Fire and plenty of common defenses Azelf when Equake incoming), Electivire (Flying, and Motor Drive, switch to Azelf when Earthquake incoming), Alakazam (just dishes it out big), Metagross (absorbs like crazy, switch to Heatran when fire incoming), Tangrowth (Takes out Gyarados and Garchomp with ease, can Sunny day to rid weather, takes all physical hits that cant be absorbed by team), and any dark pokemon, like Drapion.
They all sweep and take well, making them a reliable team.
You could add a flying type, like Gyarados or Salamence since Azelf dies most of the time, and the team is plenty weak to ground.