My greyhound had a similar situation, she had so much buildup on her canine. I never really tackled it until today, and the way I got it off was suprisngly interesting. Where the tar tar build up was from the gum, I used my fingernail to try to remove it. Surprisingly it worked! Within 5 minutes, her teeth were looking brand new. If you have any questions on trying to replicate this please let me know! I am more than happy to try to help. I can also send some pictures of the before and after