This achievement is obtained by looting 100 containers fully over time which should come naturally while you progress through the game. Lootable containers and other items will always have a gold lootbeam shining from the top of it.
The best place to do this in my opinion is New Sérène. There are tons of crates and chests littered throughout the city. The crates reset often allowing you to loot them again. Make sure you loot everything from the containers. It’s best to hit
to ‘take all’. Something that the achievement description doesn’t mention is that looting corpses and other things in the world count towards this achievement. It popped for me when I was looting a dead body just north of New Sérène by a windmill.