Scale is tough to get rid of. I had it on a weeping beech so I understand your frustration. There a way though. You’ll need to use a soil drench of Bonide’s Annual Tree & Shrub Insect Control. Follow the directions on the package carefull but you’ll dilute the liquid with water and soak the soil at the base of the tree using as much as the directions indicate. The lilacs will absorb the insecticide and it moves throughout the plant killing the scale as well as their offspring that hatch later. It worked great for me…one application did the trick but it does take some time to start working so be patient.