Aliens: Fireteam Elite

This achievement can be grindy depending on how many rep scrip you used buying challenge cards. If you're like me, and did just that, this solution can save you a lot more time (or at least some)

Although the game does not explicitly state it, any challenge card that gives a boost to credits will also boost your rep scrip by that same amount. Each difficulty gives a different amount of rep scrip, but that amount is the same across all missions only to increase as you go up in mission difficulty (casual, standard, intense, extreme and insane). If you, fellow achievement hunter, do not believe me, you can try it out for yourself. Do a mission on any difficulty, look at the end of mission rewards and look at the amount of rep scrip you get. The example I'll use is on standard difficulty, you get 50 rep scrip upon completion. Using a card that gives 2.5x credits will make that 50 increase to 125. Hidden caches do give some of the rewards for these cheevs, however, there is eventually a point where they don't give any more of so you'll have to use scrip to get more.

Hats are fairly cheap and can be earned randomly by opening caches in missions, or if you find yourself close to or near having 20, you can go to the guy that sells guns and other weaponry. Ask him about his special stock and you can buy hats for cheap. This currency can mostly be earned by doing your daily and weekly opportunities.