In order to get this achievement, you need to buy all of the upgrades on the "Training" tab (The one with the arrow with a circle around it, the right-most tab). There are seven upgrades. Two of them cost 3 medallions, two of them cost 4 medallions, two of them cost 5 medallions, and one of them costs 6 medallions. Assuming you don't make any in-app purchases, there are only 32 available medallions in the game. This means you need to collect all of them except for two.
32-30=2 left over medallions.
To do this, buy the Intel Pack for 5,000 CP's every level, except for the levels which had 0 medallions in them.
Levels 4, 13, and 28 have no medallions in them.
Levels 6, 25, 26, 29, and 30 have two medallions in them.
All other levels have one medallion in them.
Then, just go to every green circle on the map. Most of them will be dog tags, which will help you get this achievement:
And some of them will be Mega Death Weapons, which will help you get Command Points, but the ones we want are the gold medallions.
Three different things that are green on the map:
1. Dog Tags
3. Golden Medallions
If you've missed any medallions and need to go back to a previous level, go to the main menu, then click missions, and go to the mission you need.