This achievement was believed to be glitched, it is not.
When on the Targets page, you must kill each target
AND complete the "optional" objectives. The things most people are missing are the Terranova Intel Fragments and the Monkey Moonshine Kiosks.
Maka has made a guide for both of them:
Here is a link to a page with all 20, with maps and photos of the Fragments:
However, while looking around for possible guides, I found this:
It doesn't have the intel or stalls but it does have all the locations of the other optional objectives, all weapons & gadgets as well as the Agent DNA.
These aren't needed for the achievement but are either linked to other achievements or at the very least, can sate our completionist lust.
*** Spoiler - click to reveal ***You can find all weapons and most gadgets at the end of the game before the last boss fight, so if you're missing some, you can grab it there.