MLB The Show 25's new Diamond Quest roguelike mode is super addictive. It captures that classic roguelike loop that makes you say to yourself, " I understand it now, I'm gonna beat it next time." Four hours later, you're sure you can really do it if you play just one more.
The mode is fun, a great way to practice various game skills on all different difficulties, but it's also a great way to make stubs. It's possible to earn 10-12 packs in roughly an hour's time, an amount you won't get from any free mode in the game. The rare and epic rewards received from stadium challenges are also sellable. It's as close as anything I've discovered so far to an infinite stub earning scenario.
You're investing time, but 10-12 packs for an hour's grind is as good as a deal as you will get in this game. Many of them are silver and bronze packs, but they can yield diamond cards. Sell the ones you don't need, and slot the ones you do into your lineup.
If you're good enough to play on higher difficulties such as G.O.A.T, insanely profitable runs are possible. Even sticking to All-Star rank (the default for early online play), you can expect to at least pull a Ballin' Is A Habit pack.
While defeating the various challenges along the map, you'll unlock multiple boosts to contact, exit velocity, and pitching control making those final boss battles a lot easier, at least in theory. Give Diamond Quest a try, it's one of the best things about MLB The Show 25.