To get this achievement, you need to be on a (semi) collectible hunt, and specifically looking for the dead marines dotted around the level, that you interact with, pulling what looks like a key out of them in the following animation.
For every marine you do this to, you get a "key" which will allow you to unlock a bonus within the Praetor Suit. To find the suit unlock section, pause the game (using backup, not start) to get to the map, and then use RB/LB to tab across to the section.
There are 5 categories to choose from, and if you want to unlock this achievement as quick as possible, it makes sense to focus on just one category. Given you need to find the collectibles to get this, it probably makes sense to upgrade this section first (second one from the left), that upgrades the map, and helps highlight collectibles. You have to unlock 3 tiers, with the cost going up each time. You will need to find about 7 "keys" to fully unlock a category.