UPDATE
As of the June Update, all 15 cards from your deck will be put into play when you start playing. No more picking your next card at the start of the next level.
Multiple purchasable Cards can be found around the levels all costing 500 Copper each. All you need to do is now buy an extra 10 Cards found in the levels.
There are also the Free Cards in the levels as well.
-------‐----------------------
ORIGINAL POST
You need to activate 25 Intel Cards in one game, through out one playthrough. This HAS to be done with other players due to how they haven't allowed you to gain achievements on Solo Campaign. This is between all players in the game, but with my instance I pretty much got it by myself activating 25 Intel Cards.
Start from Act 1 and make your way through the game picking a card at the start of each level, activating the free Intel Cards you find & also buying cards you find through out the levels.
Randomly spawning in the levels you'll find either a open Intel file case which are free Cards or a closed Intel case which are purchasable Cards. Closed purchasable Cards are cards that aren't in your Deck.
I made a Deck starting off with the four Copper cards I have so I was racking in Copper as it was collected through out the levels.
The way I managed to get this was rather unusual... I used the 'Quick Play' method to join a game, selected Recruit and Act 1. It found a game at 'Special Delivery' and after I got to choose my character, the game told me it's taken my first four Intel Cards from my Deck I made, and then gave me the chance to pick another 16 cards. 16!!
So from there I picked up any free Intel Cards I could find in the level and then scouted every where for Copper so I could buy any purchasable Intel Cards in the level. I marked the locations with the Marker (LB) so my team mates could pick them up/buy them as well.
The achievement popped when I got back to my Fort Hope.
One of my friends said the achievement randomly popped for when she was playing with friends. She assumed one of them found Intel Cards during the game and them picking up the card counted for the team.