In order to get Seriously 4.0 you are required to max out five skill cards. This achievement only requires one though so I'll tell you exactly what you need to do for the cheapest way possible to max just one card.
Three different tiers of skills exist, below I'll list them, their color and the scrap cost to create a card of that specific tier and then the amount of scrap you get for destroying a specific card of that tier:
Common - Green - 40 - 5
Rare - Blue - 100 - 20
Epic - Purple - 600 - 150
Next I'll list the amount of cards needed for each level and the total amount of cost you would need if you were crafting all the cards for that level for a common skill:
Level 2 - 1 card (40 scrap)
Level 3 - 3 cards (120 scrap)
Level 4 - 5 cards (200 scrap)
Level 5 - 10 cards (400 scrap)
Ideally you'll want to craft or obtain common horde skill cards. If you're having no luck obtaining cards for a specific skill you want to upgrade (I suggest only upgrading skills you want and will use to help you in horde) then don't feel bad for destroying skill cards you don't want. If you were only to craft cards for a single skill due to bad luck it will end up costing you 760 scrap.
The last thing to go over is how you go about getting packs. Play whatever mode you wish to earn credits and then use those credits to buy horde packs. Horde packs are the best for this because they may give you the skill cards you want so you can save some scrap and while other packs exist for horde related cards, I suggest buying the cheapest one as you have no guarantees you'll get Legendary grade items in the more expensive packs. If you're also playing horde then you may get bounties from these packs that you can use but if it's bounty you know you can't complete then scrap it. Completing bounties will get you XP or credits, both of which can help you get more packs. If you also have character/weapon skins or emblems you're not using than scrap them too, you can always get those again in another random pack. Promotional items can not be destroyed for scrap.
If you want some more info I haven't gone over feel free to read the great guide by
Pra1s3Th3Sun: