Yo, bro, I'm gonna sit this one out.
Dude, during that last round, I hit hard. Did you see what I did? Did you see me whip out Fire Blast on that guy? Dude sure was shocked that a Normal-type like me could whip out a massive Fire-type move like that. I even think I left him with a burn!
Imma take a nap now though. Let me know when it's my turn again.
Some Pokémon are defined by their ability. Regigigas has Slow Start, Zangoose has Toxic Boost, Shedinja has Wonder Guard, and Slaking (slay-king? slack-king?) has Truant. Every other turn, our buddy Slaking sits out and doesn't take a move at all. Seriously, no matter how hard you push him, he isn't going to do a thing. It's like recharging after Hyper Beam but instead you need to recharge after Yawn.
This would be relatively unremarkable if Slaking wasn't tied with Regigigas for having the highest base stats of competitively legal Pokémon. Summed up, they both come to 670. There are some Pokémon whose base stats total 680 but you won't be able to include any of them on your team for the upcoming contests (sorry, Ho-Oh!). Most likely this is why Regigigas and Slaking have been hamstrung by "abilities" that actually negatively impact their performance.
Slaking is another one of those "odd evolves." He starts out as Slakoth, a sleepy little sloth guy who also has the Truant ability. In my mind, sloths are defined by their long, thick claws and unique face shape. The next step in evolution is Vigoroth (unrelated to Vigo the Carpathian). This one is still sloth like, but his ability changes to Vital Spirit - the guy can never sleep. Then, when you've evolved to level 36, you go back to having the Truant ability, but Slaking has turned into . . . an ape? It's not for me to question the great minds at The Pokémon Company or Game Freak, but have they never been to the Museum of Natural History? There are all kinds of sloths to choose from! Super mega sized ancient sloths who walked the primeval plains (e.g., Mylodon) even. Oh well, so we end up with Slaking, the Lazy Ape.
My first encounter with a Slaking was in Pokémon Emerald. I didn't look him up before battling him the first time and I got my ass squarely handed to me. If you don't leverage Truant to your benefit when battling Slaking he will knock you down right quick.
Why go gaga for Slaking? His base Attack stat is in the top four (tied with Regigigas and second only to Rampardos for competitive Pokémon). His main difference from Regigigas is that he has bumped up base HP while Regigigas distributes those extra base points across Defense and Special Defense. Even better than that, Slaking learns ALL the moves. Well, not all the moves, but a huge number of them. Since Slaking is pure Normal-type, you can select from the highly powerful Normal-type physical moves (Facade, Retaliate, Giga Impact, Body Slam, Crush Claw) and get a STAB bonus. Of course, he can learn a wide range of other powerful attacks, so you probably want to select from among those in a way you think is most appropriate to cover a good range of attackers (I would recommend against any of the Fighting-type moves since they don't give you any benefit against Ghost-types). Blizzard, Solar Beam, Thunder, and Fire Blast are your contenders here.
Slaking's weakest point is his base Special Defense stat. One tactic you might choose could be to get IVs in Speed and Special Defense and EV train up sharing between Speed and Attack. If you can get Slaking's speed up to 328 you may just be able to hit your enemy hard enough to cause them to switch out on your Truant turn. Possibly set him up with a Jolly nature?
Another way to help out Slaking would be to pair him with a strong defender - someone who can use Safeguard, Light Screen, or Reflect for a few turns to help a Slaking out. Possibly someone who can alternate between Iron Defense and one of the group defending moves. Any recommendations?
I am also curious to see how Slaking would operate in a rotation battle. Can you rotate him out for his Truant move and then rotate him back in when he's ready to attack again?
I'd avoid any hold items that are going to harm your Slaking (e.g., Life Orb) and focus on things that will improve accuracy (Wide Lens) since the high power moves tend to drop a few accuracy points. The Choice items will, of course, work well if you tailor them to your strategy.
I would like to see Slaking used successfully in competition. I'm certain there is someone out there who can devise a strong strategy around his Truant ability.