(This might get easier as we get closer to the end of the playoffs, as of right now, there aren't that many Galaxy Opals at the moment, I'll update when there are easier cards)
To earn this achievement you must earn a card of each gem color
-Amethyst: Best ways, All stars in Current Domination, Token rewards, Auction House, Triple Threat Offline
-Diamond: Complete 1 Spotlight Sim Challenge team.
-Pink Diamond: Complete East or West for Pascal Siakam or Brandan Ingram
-Galaxy Opal: Complete all Spotlight Sim challenges for GO James Harden
(You need 10 players from all 30 teams for spotlight sim challenges.)
If you wish to collect the cards with MT only, and hate playing online, it's best to start with Domination on the hardest difficulty to get 3 stars.
-To make things easier, see if you have, or can purchase thr Nets playbook from the auction house, when you have it, get into a game, pause, go to offensive settings, Plays, choose Pass and Screen Away, scroll down till you see Quick Thru STS. When on offense hit LB to open plays, hit LB to call Quick Thru, get into position as shown in game, when the game shows the pass button over the player next to you, pass him the ball, the passer then cuts to the basket and usually has a clear lane, pass then score inside.
If your done with Domination, Triple Threat Offline has better rewards as you win more games and the rare rewards from the Vault. You can use Spotlight Evos(if you have them high leveled) to get low leveled players.
If your fine with Online, Triple Threat Online is your best bet in making MT, the more times you win in a row, the better boards you get in the ball drop You can use Spotlight Evos(if you have them high leveled) to get low leveled players.
If you're not spending money, you're gonna have to grind, especially if you're going for the GOAT collecter achievement.
Let me know if I need to add anything