[Set] Specially Defensive

 

This is the Smogon set for gen 7 and gen 8, so it's probably the most viable. With the EVs in Special Defense and the Sandstorm, Gigalith can take special attacks quite comfortably, without being passive thanks to its 135 base stat in attack, and can also set up Stealth Rock. However, be wary of Gigalith's five weaknesses and its lack of recovery outside of the leftovers.

You can use Toxic against defensive Pokemon to reduce their staying power. Protect can be used to gain turns of Leftovers recovery and Toxic damage for the ennemy. Also, it can help you safely reveal moves the opponent uses and block them on Choice items. 

Variants 

-Replace Stone Edge with Rock Blast for more pp and the ability to pass through Substitute, Focus Sashes and Sturdy.

-Replace Protect with Earthquake to hit Steel types.

-Other damaging moves like Throat Chop can be surprising for the opponent.

Source : Smogon