Truant

The one thing stopping Slaking from destroying the entire Pokémon world.

Overview

Truant is an ability which basically halves the Pokémon's usefulness. On one turn, it will use a move. The next, it will just sit there and take damage. But even with this ability, Slaking is devastating. It can one-hit KO most Pokémon on the first turn, then when it's loafing on the second, it can just take most hits without fainting. This makes Slaking capable of almost always getting two KOs on its own.

Using Slaking

Besides the obvious, there are a few more ways to use Slaking that work toward negating Truant:

Ability changers: Skill Swap, Simple Beam, Entrainment. All of these make Slaking lose its ability, and become the ultimate force ever. At this point, Slaking can stand up to Groudon and Kyogre, and due to a negative ability of its own, completely outclass Regigigas. With no Truant ability, Slaking can make full use of those ridiculous stats.

Switching: Although I wouldn't consider it the most useful, due to how risky it can be, there is a strategy where Slaking gets a one-hit KO and immediately switches out to avoid Truant. The usefulness of this can range from Slaking taking 0 damage and winning the game, to you sacrificing both Slaking and your perfect counter to one of your opponent's overpowered Pokémon, and promptly being destroyed.

Embrace the Second Turn: Giga Impact. The turn you have to recharge for and the turn Truant stops you from taking happen at the same time. This cancels out one or the other, giving a free massive hit. The problem with this is, it's only useful when used on something that Slaking can't normally beat in one hit, and it also can't be combined with switching out, and if you rely on Giga Impact, it completely negates the positive effects of using a new ability. However, if done correctly, it can be used in perfect harmony with an ability changer.

Beating Slaking

If a Pokémon with Truant flinches when it's supposed to attack, it still can't do anything next turn, due to Truant. As a result, Fake Out is the single most reliable way to shut down Slaking strategies, since it always moves first and always makes the target flinch. Of course, you'll need some way of taking it out immediately after, since Fake Out can only be used as the first attack a Pokémon uses when sent out.

Protect also works, and forces Slaking to either faint or switch out, since you'll be alternating between that and a strong KO move. Slack-Off does counter this strategy, however, unless you can KO it in one hit or out-predict Slack-Off.

In double battles, Fake Out and Close Combat on the first turn guarantee a KO before turn 3, when Slaking can first move.

Durant

Durant's Hidden Ability is Truant. While it may seem obvious that it's better to go with the non-hidden ability, it's actually the case that Durant learns Entrainment. Fail to counter this and all your Pokémon will also have Truant.

Fortunately, it's generally not that hard to counter this. Have a good Fire-type attack Durant, and if it gets Truant, switch it out and back in (abilities change to the Pokémon's own when switched). Since the ability makes it otherwise completely useless, the only real threat Durant can pose is just Entrainment. In double battles, however, this makes it deadly when used with a good sweeper. Once again, Fake Out and Protect are good counters.

So this is an underpowered ability that can be used to your advantage? Just think what else can be used to much greater effect than anyone knew! Until next time.

-Pokéxplain