100% Orange Juice

If you wish to not rig this in multiplayer:

Stack cards like Piggy Bank, Holy Night, and Nice Jingle. Play on a map in which bases are avoidable, like Tomomo's Abyss or Clover, and enable the Miracle field event. If you're feeling risky, you can play as a bully character to leech stars off of weaker characters - but playing as Aru or Poppo are significantly safer bets, due to their hypers and (usually) survivability.

If you have the Breaker Pack DLC, though, you can extremely easily stall the game on Star Breaker Episode 4 while you get 700 stars on casual difficulty. Just pick a bully character and enjoy the massacre.