It's been way too long since I've tried a POTW. Anyways...
Gigalith is your generic physical Attack and Defense-oriented Rock type Pokemon. Average HP, low Special Attack, low Special Defense, a Speed that is only usable in Trick Room, and the Sturdy ability. The Dream World ability of Sand Force gives it an edge, though, the others don't have. Gigalith is also one of the few Pokemon that is a Rock only type, meaning a Fighting/Water type move won't cause terrible damage to it. It also has a very shallow movepool, but not a bad one.
Abilities:
Sturdy: Non-consumable Focus Sash. Also protects you from Fissure, Sheer Cold, etc, but that's not as important as the Focus Sash part.
Sand Force: Powers up Rock, Ground, and Steel type moves in a sandstorm. Sandstorms also have the extra benefit of increasing aforementioned type Pokemon's Special Defense by 50%.
Stealth Rock Powerhouse or Tank? Your Pick.
- Stealth Rock
- Sandstorm
- Mud Slap/Earthquake
- Stone Edge/Rock Slide
Item: Leftovers
Ability: Sand Force
EVs and Nature: Adamant (+Atk, -SpAtk) or Careful (+SpDef, -SpAtk); 252 Atk/64 Def/192 SpDef or 4 Atk/252 Def/252 SpDef
Set up Stealth Rock and, to last longer, use Sandstorm to increase your Special Defense. This set sort of varies as it really depends on whether you want to focus on an offense or defense strategy. You could use a Calm nature and defensive EV spread with Mud Slap to make a Gigalith that can survive for a long time and forces switching with the accuracy loss from Mud Slap. In an offensive set, Stealth Rock takes somewhat of a back seat to the Sandstorm-Sand Force combo, making Earthquake hit harder and Stone Edge/Rock Slide do insane amounts of damage with Adamant nature and 252 Atk EVs. As you knock out opponents, Stealth Rock hurts the next one coming in.
Power of the Sandstorm
- Stone Edge/Rock Slide
- Earthquake
- Heavy Slam/Explosion/Curse
- Sandstorm
Item: Life Orb
Ability: Sand Force
EVs and Nature: Adamant (+Atk, -SpAtk); 252 Atk/64 Def/192 SpDef
Not too different from the offensive side of the previous set, I know. But this set takes full advantage of Sand Force by including all the types of moves it boosts and taking it even further with the Life Orb. Although, if you want to, you could replace Heavy Slam with Explosion. Heavy Slam is affected by weight comparison, so it won't always turn out to do good damage. Explosion, however always makes for a good last resort and does great damage to anything not Rock, Steel, or Ghost type. Curse, though, is another option if you want to increase Attack and Defense (with base 25 Speed, who cares about the Speed drop?).
Just Your Average Everyday Exploding Rock (Seriously, Most Rock Types These Days Can Use Explosion)
- Toxic
- Iron Defense
- Sandstorm
- Explosion
Item: Leftovers
Ability: Sturdy
EVs and Nature: Adamant (+Atk, -SpAtk); 252 Atk/64 Def/192 SpDef
Toxic, Iron Defense, and Sandstorm will make it seem like you're playing a defensive set, so they might use Taunt, expecting a switch, and then you use Explosion, destroying whatever was on their field. Of course, you can have a little fun with Toxic stalling in this set if you want.
Other Options
Not many. I usually try to make at least four sets for each POTW I do, but Gigalith's movepool is just not diverse enough.
Swagger confuses your opponent. Use it with Stealth Rock and force a switch.
Headbutt can cause flinch...in Trick Room, I guess. Not amazing, I know, but I just wanted to mention it.
Smack Down is a Rock type move with only 50 power, but, hey, it's the only Rock type physical move with 100 accuracy. Making Levitating and Flying type Pokemon vulnerable to Ground type moves is also a plus.
Gravity allows Levitating and Flying type Pokemon to be able to hit by Ground type moves. It also decreases the Evasion of everything on the field while the move it is active, so it's a viable option in Doubles/Triples.
Double and Triple Battle Options
Gigalith is well equipped in Doubles and Triples with Rock Slide, Earthquake, Explosion, and Gravity. They're all pretty handy. Low Speed means you could put it in a Trick Room team.
Partners
Flying types for Earthquake, Ghost types for Explosion (or just bring something that has Protect). You could partner Gigalith with a Trick Roomer to compliment its poor Speed. Bronzong makes a great partner because it can learn Trick Room, can Levitate to avoid Earthquake, and with Steel typing and high Defense, can take an Explosion pretty well. Other than those obvious choices, if you have a partner with low accuracy, high power moves, have Gigalith use Gravity for great results. Also try partners with OHKO moves (Fissure, etc.) for devastating results. Sure, those OHKO moves still won't always hit, but they'll be a lot easier to use with Gravity in effect.
Counters
Almost any Water type move (you know, cause most of them are Special attacks) will OHKO, except with a Sturdy Gigalith; so bulky Waters will be able to handle Gigalith's attacks and return with an OHKO. Grass type moves are also mostly comprised of Special moves, so they will OHKO as well. With Gigalith's base 130 Defense stat, it would take at least 2 hits from a Physical move to take it down. Entry hazards nullify Sturdy's Focus Sash capability, so they would help. A weather changer can stop Sandstorms to nullify Sand Force.
Pre-Evolution Corner
Boldore can really do anything Gigalith does, except maybe for Heavy Slam, because its weight is less than half of Gigalith's. Eviolite also makes Boldore's Defense very impressive.
Roggenrola has the same sets, has amazing Attack and Defense compared to the majority of Little Cup, and does extremely well in Trick Room. Eviolite makes this thing untouchable from a good amount of Physical attacks.