I am going to assume this is Gen VI. You will need to chain breed Sucker Punch onto Druddigon, using either Nidoran Male or Ekans as a father. Because breeding Nidoran is a pain, I will use Ekans as an example.
You'll have to teach Ekans Sucker Punch by breeding a female one with a male Raticate, Dugtrio, Furret, Mightyena, Mawile, Spinda, Kecleon, Absol, Glameow, Liepard, Furfrou, or Meowstic that knows Sucker Punch. (Or any one of their evolutionary lines, so long as it knows Sucker Punch.)
You would then have to breed a male Sucker Punch Ekans with a female Druddigon, and all the offspring Druddigon will know Sucker Punch.
Hope I helped!
