When you complete Matt's side quest "The Pledge", you're given a collectible finder. You can filter it to show only clusters. Some groups of clusters may be centered on a tower. You must climb to the top of the tower and collect them along the way. This is usually the case if you see a group of clusters concentrated in one place and don't find them on the ground. Some are in Telekinesis balls, which you must throw with the telekinesis power. Some are embedded in walls or floors and need to be pounded out by a power. Blast usually works for the walls and stomps work on the ground ones. If you've been doing this over the course of the game, you'll already have collected a lot of clusters. There are 1255 clusters, and that's a lot. Early on in your collection of them, you'll be able to see about 10-20 around you if you stand on a building. Just go to different map areas with large amounts of them and use your sprinting, jumping and other powers to rapidly collect them. As you keep going, the concentration of them on the map will gradually go down. When I reached the end of this process, I didn't unlock the achievement. I was missing one cluster, which I found later. If this happens to you, look around the map very carefully. A cluster's symbol may be almost completely hidden by a bridge or other obstacle.