Gigalith Is a heavily underrated Pokemon in my opinion because with okay HP, good sp. defence for a rock type, and monstrous physical stats, this thing should be in UU. If you know how to use it, it could wreck the OU tier to shreds. Its just really hard to use. That's because with the prevalence of super-powered Pokemon inOU, this thing has a had time, as its a modest Pokemon.
Gigalith@assult vest( turns 80 sp. defence into 120, although this set is used for offence because of assult vest's drawback of being only able to attack ) or leftovers or smooth rock( for sandstorm )
Evs: 252 hp, 100 sp. defence, 158 attack or 252 Hp/252 sp. def./4def.
Ability: sand force
Explosion: just to rage your opponent after the manage to almost kill it. Also good for blowing up Megas and blasting anything to bits.
Sandstorm/ rock slide : sandstorm if your not using assult vest, goes good with sand force. Rock slide has good power and works great with STAB and gigalith's high attack.
Stone edge/bulldoze: bulldoze slows down the opponent and gives extra coverage. Can be replaced with earthquake if you want more power. Stone edge is for getting TONs of damage from STAB, base power, and gigalith's good attack.
Stealth rock/gravity/iron head: the first two are support against flying types for your team, along with fire and bug and ice in the case of stealth rock. Iron head is for coverage.
Hoped this helped
Also, you have to play it juust right, as if you don't that's why this thing's in PU.