There are 4 ways to get a diamond player, the first 2 are probably the worst way if you just want this achievement.
Buying packs & Collector levels:
Granted 2k is releasing new diamond players, it's nearly impossible to pull one (as of March 2016 they increased the pack odds, which is how I got my diamond, but I wouldn't trust it.), and unless you're willing to spend a lot of money on packs to get at least 2,300 cards for Diamond John Stockton you probably won't be able to do it.
The 3rd way is to just buy one off the market:
UPDATE(6/1/16): Diamond Bob Cousy is currently the cheapest card on the market for this achievement, he ranges from 55k-70k, another card usually up for around the 70k price is Diamond Isaiah Thomas. Definitely the easiest way to get this achievement now, playing through domination will get you the MT plus more.
Getting diamond Kareem:
If you look at the weekly challenges there are multiple Road to the Finals challenges, as of today the Warriors, Rockets, Pistons, Lakers, Spurs, Heat and Suns are all up.
In order to receive diamond Kareem you must collect all the Finals Championship Reward set, which you complete by beating all these challenges.
Each team you need to go through 4 series and go through what each teams has gone through. For each series you will need a MINIMUM of 8 players from that team to start it. Once you win 4 games in the first series, the rest require just 6 players including the reward player you received from the previous challenge. (Requirements are shown on the game if you're confused by my explanation).
Now you can get some nice cards on the way to getting diamond Kareem as well such as Amethyst Tony Parker, Amethyst Pau Gasol & Amethyst Dwyane Wade.