It's only possible in Gen VI, where the mother has a chance of passing down egg moves as well. You will need a copy of Black 2 or White 2, and Pokebank.
Firstly, find a male Weedle in Black 2 or White 2 and evolve into a Beedrill. Then head to a move tutor to learn it Drill Run. Transfer it to X and Y, breed it with a female Karrablast and the resulting offspring should be a Karrablast with Drill Run.
Then, find a male Veinpede and evolve it to Level 30. Take it to the Move Relearner to learn it Megahorn. Breed it with another female Karrablast and the resulting offspring should be a Karrablast with Megahorn. The Karrablast with Drill Run and the Karrablast with Megahorn must be of different genders, so make sure of that.
Then, breed your two offspring and if all goes well, you will have a Karrablast with Megahorn and Drill Run. The mother can pass down egg moves but it's a smaller chance than the male passing down egg moves, so keep trying and hopefully things will go well.
Hope I helped. :)
Source: Lots and lots of Serebii