What strength peroxide are you using? If the skull is free of grease, just submerge it without simmering. Let it sit overnight at least. If you get it into the right sort of container (like a shoebox), you can cover the skull with peroxide almost up to the antler burrs. Cover the rest of the top of the skull with white paper towels and get them under the burrs but not touching where the color starts. Keep that wet with peroxide and the whole skull will be whitened and you will not have to be retouching the antlers. Done properly, with heat, pasting can also work great and uses a minimal amount of peroxide. You get to keep using what you have for a long time.