First off, I have to ask: Who the heck are you battling? Who uses Slaking?
But, if he's a major worry for you, the strategy you mentioned is one of three major strategies for taking out Slaking.
1 – Alternating Protect on the turns Slaking can attack and attacking on his Traunt turn.
2 – Flat-out OHKOing with a powerful Close Combat or Low Kick.
3 – Doing what you suggested; Dig/Dive/Fly on his attacking turn and striking on his Traunt turn.
Of the above methods, #2 is probably the best. However, the two-turn-attacking method works pretty well.