Slaking, Pokémon's answer to Donkey Kong, if Donkey Kong wasn't motivated by anything. Nothing motivates Slaking beyond Mummy, Worry Seed, Simple Beam, Skill Swap, and Neutralizing Gas. And that alone wrecks Slaking's viability. Being able to only act every other turn enables Slaking's opponent to freely play around Slacking, from Protect utterly walling Slaking out to numerous setup opportunities, Slaking is brought down to the lowest lows by its ability.
One Big Punch
Slaking
Ability: Truant
Item: Normalium Z
-Giga Impact
-Return/Retaliate
-Earthquake
-Hammer Arm/Night Slash
Nature: Jolly (+SPE, -SATK) or Adamant (+ATK, -SPE)
EVs: 252 ATK/252 SPE/4 SDEF
Slaking Possesses one of the most destructive attacks in the game, a Breakneck Blitz powered by Giga Impact hits 300 base power, after STAB and no normal type in the game has as great of Attack as Slaking.
252 Atk Slaking Breakneck Blitz (200 BP) vs. 244 HP / 52 Def Gliscor: 301-355 (85.5 - 100.8%) -- 6.3% chance to OHKO
252 Atk Slaking Breakneck Blitz (200 BP) vs. 0 HP / 0 Def Zygarde-Complete: 324-382 (56.5 - 66.6%)
252 Atk Slaking Breakneck Blitz (200 BP) vs. 252 HP / 4 Def Toxapex: 264-312 (86.8 - 102.6%) -- 18.8% chance to OHKO
252 Atk Slaking Breakneck Blitz (200 BP) vs. 248 HP / 212+ Def Tangrowth: 243-286 (60.2 - 70.9%)
252 Atk Slaking Breakneck Blitz (200 BP) vs. 252 HP / 112+ Def Landorus-Therian: 336-396 (87.9 - 103.6%) -- 25% chance to OHKO
-1 252 Atk Slaking Breakneck Blitz (200 BP) vs. 252 HP / 112+ Def Landorus-Therian: 223-264 (58.3 - 69.1%)
252 Atk Slaking Breakneck Blitz (200 BP) vs. 252 HP / 200+ Def Rotom-Wash: 273-322 (89.8 - 105.9%) -- 37.5% chance to OHKO
Several Pokémon known and relied upon for their defensive abilities, and all of them are maimed by Z-Giga Impact. Any tank that is weakened and doesn't possess a resist and anything else not packing an immunity to Normal moves is going to be swiftly KOd (bar Sturdy/Focus Sash). Return/Retaliate is for reliable STAB. Slaking makes a great revenge killer and a base 210 power after STAB is nothing to joke at in regards to Retaliate, while Return is more stable. Earthquake wrecks Steel and Rock types, while Hammer Arm hits those that Earthquake doesn't, and Night Slash hits Ghosts.
Don't ever use Giga Impact without the Z-Move. Any gains you get from knocking out a 2nd pokémon go away when you just gave your opponent a free turn to do anything they want (as you can't even switch unlike normal Truant).
Other Options
Encore -Force your opponent into a move for a few turns and prevents Slaking from being utter setup fodder