I think the best move you can attack Cofagrigus with would be Aerial Ace, but with Slaking's power that'd be dangerous.
The next best thing would be to have a faster, lower-power non-Ghost Pokemon hit it, and then have Slaking use Frustration on that Pokemon (make sure Happiness is maximum).
Slaking isn't KOing cofagrious anytime soon, Aerial Ace or w/e.