The exact opposite.

[99bottles]

Source: I'm writing this.
Context: Dialogue.

-I would kiss (person A) and slap (person B).
-I would do the exact opposite.

Does the exact opposite make it clear that I mean she would kiss person B and slap person A? Or should I word it differently?

 

[Yankee]

Source: I'm writing this.
Context: Dialogue.

-I would kiss (person A) and slap (person B).
-I would do the exact opposite.

Does the exact opposite make it clear that I mean she would kiss person B and slap person A? Or should I word it differently?

Yes.

 

[Tarheel]

I suggest:

Alice: I would kiss Mike and slap Pete.
Barb: I would do the exact opposite.

Barb would kiss Pete and slap Mike.