As Marc says, using the aperture ring will cause the flash to fire at full power. Because the aperture is set, the only variables are shutter speed and ISO. Of course being flash, the shutter speed is also set. So ISO is all you have to play with.
You can not win in this situation. Your only option is to set ISO100 and a (much) higher F-Stop, and live with the fact that the background will be black. It won't be a great photo but at least your subject will be properly exposed.
The 'proper' way to address this situation with a better flash, would be to pre-set your ISO, F-Stop and shutter speed to underexpose by a stop without a flash. You would then switch to flash and dial back the output until the subject was properly exposed. Don't worry too much about camera shake as only the backgroud will be affected.
Of course, if you had that nice flash, you could set it up to fire through a soft box at a pleasing angle.... but this is how photography turns into an obsession