It will work for a time.
Most teams in carry a Fighting-Type Pokemon and will send it out when they see this unstoppable, rampaging beast sweeping their team. With 65 Special Defense and 100 Speed, a Special Fighting move will be its escort into the next life.
The only moves like that are Focus Blast, Aura Sphere, Vacuum Wave, Final Gambit, and Secret Sword (which deals Physical Damage anyways). Vacuum Wave has priority so any Pokemon that knows it can be faster than Slaking as long as Slaking doesn't use priority, and Final Gambit does damage equal to the user's HP, so that won't matter.
Here is a list of Pokemon that know Aura Sphere and/or Focus Blast and are faster than or equal in speed to Slaking:
Aura Sphere: Mewtwo, Mewtwo-X, Mewtwo-Y, Mew, Palkia, Mienshao
Focus Blast: Charizard, Charizard-X, Charizard-Y, Raichu, Alakazam, Alakazam-Mega, Gengar, Gengar-Mega, Electabuzz, Typhlosion, Miltank, Sceptile, Deoxys, Deoxys-S, Deoxys-A, Infernape, Floatzel, Lopunny, Weavile, Regigigas, Darkrai, Arceus, Victini, Simisage, Simisear, Simipour, Archeops, Zoroark, Cinccino, Accelgor, Cobalion, Terrakion, Virizion, Tornadus, Thundurus, Landorus, Keldeo, Helioisk, Hawlucha, Noivern
With a choice scarf, more than three-quarters of the Pokemon on these lists would outspeed Slaking.
So, yeah. Your strategy will work for a time, but can easily be shut down.
Source: Above links and Slaking's Pokedex entry.