the way you only compress the louder notes is by playing with the threshold while looking at the gain reduction meter. you set it high enough to not induce gain reduction on most notes, but when the high volume note hits, it goes over the threshold, and triggers the gain reduction, which will be visible on that meter.
or you could just set it to like 4:1 at -20 for like 12 or so db of gain reduction on the whole line and just slam it to hell.