The only gold, alien species unlocks I have gotten were from Veteran level packs, but thankfully they are not very difficult to get the money for, even on Bronze.
The best strategy is to play with just three players (though this might be difficult unless everyone is about level 5) when you are starting out and roll through Bronze. Spend an hour doing this, at about 15-20 minutes per mission, and you should be getting 14-17k credits per mission.
At that point, buy two veteran packs and two recruit packs (for weapon consumables, equipment upgrades, etc). You should be level 7-10 by now. Rinse and repeat for another hour or so, and buy all veteran packs. By that time you will likely have unlocked a few good guns and class/race combos and can move up to Silver with MUCH greater ease.
Human classes, in general, are difficult until the teens to take into silver without good players and a good team.
Exceptions exist for the more OP classes, though-Engineer, vanguard, infiltrator, etc.
I unfortunately continue to have horrible luck with unlocks, but the methods above got me the few gold cards that I have. Across three platforms I have only unlocked 4 total race/class packages, never gotten the Mattock, Revenant, Widow, or Carnifex.
So I am cursed in summary!
