Moves
Stone Edge allows Gigalith to use Continental Crush, which takes out common Pokemon like special or Flyinium Z Dragonite, Mega Charizard X, and Porygon-Z. Laser Focus lets Gigalith beat Defense-boosting Pokemon like Clefable and Kyurem. This also lets it force Taunt + Z-Reflect Tapu Lele into a mind game. Curse lets Gigalith set up against Pokemon that don't always get KOed by Continental Crush, like Flyinium Z Dragonite. It also lets it take a second hit from Pokemon like Haxorus. Earthquake lets it take on certain Steel-type Pokemon like Grassium Z Heatran and Laser Focus Aegislash, which don't take much damage from Continental Crush. Protect is also an option that lets Gigalith more reliably beat special or mixed Dragonium Z Dragonite and reduces the mind games against Tapu Lele. Rock Blast can also be used to beat Substitute users like Jumpluff and Whimsicott, or Counter users like Crustle.
Set Details
252 EVs in HP and 252 EVs in Special Defense along with a Careful nature give Gigalith the best chance of taking two Flash Cannons from Heatran. This also improves its odds of tanking hits from hard-hitting specially attacking Pokemon like Porygon-Z, Tapu Lele, and Mega Gardevoir. The Defense IVs are reduced to 18 to avoid giving Genesect a Special Attack boost from Download, thus granting Gigalith a better chance of beating Choice Scarf variants. The remaining EVs are dumped into Attack.
An alternative EV spread of 168 HP / 104 Atk / 236 Def with a Relaxed nature and Superpower can also be utilized to reliably beat non-Substitute Mega Lopunny, Registeel, and Choice Scarf Excadrill.
Usage Tips
Use Curse once against bulkier foes like Mega Altaria, most Porygon2, and Curse Mega Scizor before using Laser Focus, since these Pokemon need more than a +1 boost to be taken down. A combination of Sand Stream-boosted bulk and Continental Crush allows Gigalith to tank a +1 Hyper Beam from bulkier variants of Mega Gardevoir but only one from offensive variants while being able to KO any variant with Continental Crush + Stone Edge + 2 turns of Sandstorm damage. So, against Mega Gardevoir, always use Continental Crush turn 1, followed by Stone Edge, even if it uses Will-O-Wisp. Against bulky Tapu Lele, there are two options; you can either use Laser Focus turn 1 if you predict the Tapu Lele to use Z-Reflect, or you can use Continental Crush right away if you predict them to use their offensive moves or Taunt on the first turn. Avoid sending in Gigalith against defensive Pokemon like Toxic + Foul Play Porygon2 and especially against those immune to critical hits like Mega Slowbro and Type: Null.
Team Options
Grass-types like specially defensive Tapu Bulu and Ferrothorn are great teammates because they can take on bulky Water-type and Ground-type Pokemon like Mega Gyarados, Tapu Fini, Donphan, Zygarde, and Kommo-o for Gigalith, with Gigalith taking care of Mega Charizard X, Mega Charizard Y, and Heatran for these Pokemon. For similar reasons, Steelium Z Celesteela can also be counted upon as a teammate. While it loses to Tapu Fini and Zygarde, it takes care of Mega Metagross, Landorus-T, Mega Slowbro, and Aegislash for Gigalith, with Gigalith taking on Mega Charizard X, Mega Charizard Y, Incineroar, and Zeraora in return. Some Pokemon that benefit from Gigalith KOing Mega Charizard X, Mega Charizard Y, Porygon-Z, and some Dragonite variants, such as specially defensive Mega Mawile and Genesect, appreciate it as a teammate. In turn, these Pokemon can beat Mega Gyarados, Mega Slowbro, and Mega Lopunny for Gigalith.