Completed this one recently.
The achievement (as of currently) does not work if collectively as a group gather 15 samples. The achievement is individual based, you must extract carrying 15 samples yourself. (Your co-op partners can drop the ones they collected, as long as YOU leave with 15 samples you will get the achievement.)
I found it easy to complete on 'Challenging' difficulty missions, just because there is more samples to discover. There tends to be a sample at Enemy nests, Points of Interests the (?) and secondary objectives. (not always but mainly those places.)
Me and a buddy duo'd a Challenging Mission on a bug-planet. Ran around the map and collected 15 total. We extracted and the achievement didn't pop, so we did it again but this time had one of us carry the total amount and the achievement popped for them.