Simply either win 5 matches of Save the Safe or finish 20 matches.
This can be done alone with one or more controllers signed in as guest. From the Main Menu, select Play and then Create Match. Set up a Private Save the Safe match, press to sign in your guest account(s). Press to change the game settings to your preference. If you want to knock this out quickly choose $3000 to win, skyscraper and earthquake power-up as the only power-up to spawn in the match. Rinse and repeat this until you get 5 wins.
You should also get "Inseparable" while going for your wins. See "Inseparable" for more details.