I would strongly recommend for people to upgrade common cards, like Build cost, or Health boost to name a few, because that will require the least amount of scrap to craft, and the most common skills that come out of Horde Booster packs. Horde booster packs are a good way to start, as they provide many common skills, and the highest chance of duplicate common skills.
Please be aware you need:
1 card out of a pack to be able to craft duplicates, like a Operations pack or a Horde booster pack.
You need:
1 duplicate to upgrade to level 2,
3 duplicates to upgrade to level 3.
5 duplicates to upgrade to level 4.
10 duplicates to upgrade to level 5,
20 duplicates to upgrade to level 6.
Overall you will need 39 duplicates of a single card, which with common cards add up to 1560 scrap, times by 5, to 7800, at a maximum, to craft your way from level 1 to 6 for 5 cards, and get your achievement. It should pop a few seconds after upgrading your final card to level 6.