Best way to earn stubs without grinding the market?
Depends, if you are good at the game, Battle Royale.
Getting all the event wins gets you a lot of packs + some players, all together that's a decent chunk of change.
I've made about 20k today doing showdowns for affinity, it's fun to do if you don't want to play online.
1st Inning Showdown is an easy stub maker.