You can't; its ability Truant causes it to stop attacking every other turn. However, if you are Double Battling then you can have a pokemon use skill swap on slakoth/slaking to switch its ability with the other pokemon.
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Sort of but it's otnthe best Pokemon to have. Sometimes if you tell it to attack it will loaf around or go to sleep. If it were up to me I wouldn't catch one.
In Ruby/Sapphire/Emerald the chances of cathing a Slakoth is 5%. Just walk around Petalburg Woods for a while, you'll get one eventually.
You can't. If the Pokemon is loafing around(Slakoth and Slaking) then that just means it has the ability Truant. That's just how it works. You can, however, pair it with a Pokemon that knows skill swap. Then, the slakoth or slaking would move every turn. Unfortunately, the other Pokemon wouldn't.
because palkia and dialga weren't around that early of pokemon emerald. they are newer then emerald
They are Pokemon in certain routes that have their appearance percentage raised so there are more of them around.