Using ND filter is one possible way, but you'll need to check the shutter speed regularly and not letting it dip below the 1/focal length rule of thumb. And the limitation is, since the usual max sync speed is 1/250s, the shutter speed margin becomes rather narrow. This is especially true when you're shooting portraiture at, say the 200mm end of a 70-200 lens, which you only have 1/200-1/250s to work with. To overcome that, a monopod can be used to help in stability. Using HSS would not have this issue but you'll need to watch the shooting distance, since the smaller aperture you use, the closer the flash need to be to the subject.