Foul frequency is mostly for those "borderline" swings with how close to the edge they can be and still produce a fair ball (or contact at all). If a swing is "clearly foul", it's just going to be foul. Likewise if it's "clearly fair".
I have at 0 for both sides also and get these kinds of "grinder" PAs sometimes - both for/against me.
Most of the time I start throwing significantly out of the zone away, in, or up (down, too, if no one on base...don't want to risk a WP otherwise and pitches in the dirt are more likely). Either he'll chase something and strike out, I'll (accidentally) hit him, or it becomes an "unintentional intentional walk" and I'm done with him in less pitches, trading pitches saved for a walk.
And if he happens to get a hit/put it in play somehow, that's fine, too.