It takes 204 stars to get to this milestone on your way to general. While the daily challenges seem like an easy way to get stars, I recommend checking out the medals for the current operation. There are tons of easy ones that just require you to play the game in any mode.
As an example, just winning a single ranked match of each mode gives 5 stars each... winning 10 rounds of each ranked mode gives 5 stars each (even if you lose matches along the way). *Note: These are subject to change for each operation*
Horde and Escape seem a bit more grindy, but they are easy as well.