Slaking
Slaking can either be a dead weight or an incredible powerhouse for a team. It's ability is the biggest thing in question; Truant only allows it to attack once every two turns. If you can remove it, you quite possibly have a powerhouse on your hand; otherwise, Slaking is mostly a dead weight every other turn.
[SET]
Item Held:Choice Band
Nature:Adamant (+Attack, -Sp.Attack)
EV's:252 Attack - 252 Speed - 6 HP
-Giga Impact
-Hammer Arm
-Pursuit / Shadow Claw
-Earthquake / Rock Slide / Ice Punch / Aerial Ace / Thunderpunch
[SET COMMENT]
Giga Impact was created for this Pokemon. Slaking has an ENORMOUS Attack (it can reach 460). Now, Giga Impact has 150 Base Power, 1.5x by STAB and 1.5x by Choice Band = 337.5! And of course, the best part is here: Slaking would lose the next turn anyway, so Giga Impact has no drawbacks on Slaking. Giga Impact is going to be your main attack, but it is weak against Rock and Steel. Hammer Arm will do the job here and don't be afraid of the Speed reduction because Slaking is a Choice Bander and you will switch it out a lot. Giga Impact cannot hit Ghosts, so the third move should be Pursuit or Shadow Claw. Shadow Claw is usually more powerful than Pursuit, but Pursuit will be more useful because it will finish a lot of Pokémon who try to escape. The last move depends on what you want to hit. Earthquake is a great sweeping move and will surely defeat Heatran. Rock Slide will defeat Flying-type Pokémon. Ice Punch is for the Dragons. Aerial Ace is for Heracross and Breloom and Thunderpunch is for Skarmory and Gyarados.
[ADDITIONAL COMMENT]
Using a different moveset on Slaking means that you are insane. Slaking's Ability doesn't let it do anything else and as I said before, if you want to use Giga Impact on a Pokémon, it should be Slaking due to its Ability. Choice Scarf could be used for more Speed, but Truant cuts the quick sweeping capabilities, so Choice Band is better in my opinion.
[Other OPTION]
As a person I don't like big, fat, silly guys that the only thing they do is punching others. Although Slaking is exactly like this, I cannot deny that he is a very powerful Pokémon. The new generation improved Slaking's moveset by giving him Hammer Arm, Pursuit and the Elemental Punches. Slaking's power is almost unstoppable and can OHKO many Pokémon in the game. Its ability is obviously his weak point but it is not THAT bad with the Choice Bander set or when Slaking uses Giga Impact. If you want sheer and brutal power, Slaking should definitely be a part of your team.
[CHECK AND COUNTER]
Protect and a faster Substitute make Slaking useless.
Pokémon with Intimidate can switch on the turn that Slaking loafs around and cut its Attack. If Slaking uses Earthquake on your Pokémon, you can then send out Gyarados or Salamence. Slaking will surely change because it cannot hit you, and you will have a free turn.
Skarmory and Bronzong can take some hits, but they should be very careful.
Stealth Rock, Sandstorm and Spikes will always be good, because Slaking is going to switch a lot.
Slaking is a Pokémon that uses a lot of prediction and to counter it you will need a lot of prediction too. If Slaking uses the right move, nothing can resist its power. You should take advantage of its Ability and predict some hits to defeat it.
