Edit: For some reason I saw/thought of Slacking and then proceeded to write about Slacking the entire time. Somehow I didn't even check once. But pretty much everything I said is also true for Regigigas so whatever. I'm dumb @[email protected]
This is a variation of hundred similar things. If this happened perfectly and uninterrupted then, possibly just through surprise (though someone would probably instantly know what you were trying) then it could. There'd be a couple problems though:
Cofigrigus has niche and is a good disrupter with WoW and Mummy but it's not a great Pokemon by any means. It has trouble with it's defensive role as it lacks real synergy and is difficult to fit onto a team without being a splash on or deadweight.
As I mentioned, it's not that hard to see through this strategy. You'd essentially have to waste a turn with slacking and Cof first, meaning your opponents could just gang up and beat Slacking turn one. Unless they're running Stall, they can easily 2HKO Slacking with whatever combination of sweepers. They risk their sweeper getting burned or Mummy'd but it's better than them losing, and if Slacking is you're main wincon they're still at an advantage. Even then, moves like Fake Out are very common place, meaning it could take more than one turn for slacking to set up. And THEN there are mindgames with the super common Protect, essentially wasting Slacking's useful turns.
Slacking also hits hard - even a weak move would dent Cof, meaning it's significantly easier to beat Cof too. If your opponent has any wits about them, they can easily gang up on Slacking and then beat the weakened Cof next turn.
Lastly, though this is a relatively small point, being forced to run a weak move on Slacking is kinda just wasteful. Slacking has good moves and coverage, restricting that to essentially three slots sucks.
So...I wanna say no. It's pretty fun and I'd highly suggest using it low ladder, it'd probably feel awesome if and when it works. But to take seriously? I wouldn't bother.