To get a Larvitar with both Assurance and Stealth Rock:
Step 1: Assurance
In order to breed Assurance, breed a female Larvitar with a compatible male Pokémon that knows Assurance. The offspring should have Assurance as an egg move.
Step 2: Stealth Rock
If you are playing ORAS, there is no need to breed Stealth Rock onto Larvitar. You can have a Move Tutor teach it for 8 BP in the Battle Resort. If you insist on breeding, however, repeat Step 1 until you have a female Larvitar with Assurance, then breed it with either a male Tyrunt or Tyrantrum that knows Stealth Rock. The offspring should be a Larvitar that knows both Stealth Rock and Assurance.