As for removing Truant, that doesn't make Slaking suck any less, as there is no way to remove it without going through a ton of hurt. Double Battles you are using up that turn for Gastro Acid. So one Pokemon is effectively useless that turn. Well even more useless then what all Gastro Acid users besides Amoonguss already are. That Amoonguss is then giving up its moveslot for that Gastro Acid. Meanwhile, the foe is going to focus down Slaking, which is going to be pretty easy to do. So in Doubles, Slaking is far more trouble than it is worth.
Now for the Cofagrigus, Mummy, Slaking switch in, Pursuit trick, that's also gimmicky as hell and completely useless. First off, you're having to make Cofagrigus take a physical hit. And then switch out right away. So not only are you damaging Cofagrigus, but you're also risking a lot of health from Slaking with the switch in. Of course your opponent can then switch out right away after getting Mummied. Then Slaking won't be able to pull off Pursuit. Even if you do manage to successfully pull it off, what does that leave you? A damaged Slaking that won't be able to switch out without getting back Truant.