For the most part, this achievement is really easy to unlock. Unlike Crackdown 1, this game lets you know exactly how many of each orb is within a given district of the city. The Orb Finder, used by pressing Up on the D-Pad, will show you where any Agility, Hidden, or Online Orbs may be hidden within a certain distance. It'll show colored dots for a second and takes about five seconds to recharge.
To see more of the map while searching for more orbs, I used the Agency Helicopter over a certain area and boosted by clicking the Left Stick. It's a bit tiring to do but you're allowed to see more of the surrounding area on your tracker. The Helicopter is available after hitting Level 5 Agility. If you plan on just going one island at a time, you won't hit Level 5 Agility so you'll have to pick up a few orbs from another area (If memory serves me well, Level 5 Agility unlocks around 250+ Agility Orbs....this also includes three Rooftop Races and a handful of Agility Renegade Orbs).
Now, for the actual orbs:
These are the most abundant of the orbs but they are also the easiest to obtain. Given the rather muted color palette of the game the glowing green orbs should be easily visible. These will be out in the open and usually at the highest point of any given structure. Hope Springs has 231, Green Bay has 140, Unity Heights as 115, and Agency Island has 14.
Given that the Agency Island isn't really complete until all the Beacons have been activated, it may be best to just save that island for last and progress through the story. However, it is also possible to just fly a Helicopter a decent height over an orb and to jump from the Helicopter and onto an orb.
These will show up blue on your tracker and are the second most common orb. Hidden orbs are different from Agility Orbs in a few regards. Firstly, Hidden Orbs are, obviously, hidden. A few may be out in the open but many of them will be inside the broken parts of buildings or in dark areas underneath highways. Secondly, Hidden Orbs may be hidden behind glass. The glass works in a goofy way that actually prevents you from being able to see it. Thirdly, Hidden Orbs can be obtained while inside of a helicopter. So, if you see a Hidden Orb on top of a building or a small platform, instead of jumping on it while on foot you could just fly the helicopter over it. This is useful for getting some of those orbs that are ridiculously high in the air.
There are 87 in Hope Springs, 101 in Green Bay, 105 in Unity Heights, and 7 on the Agency Island.
Online Orbs show up gray on your tracker. Given that your tracker is gray it can be a little difficult to see these on the map. Also, they can be hard to see in general. In order for these to be captured, another player must be close enough to them for them to activate (they will turn green and eventually be captured when someone else gets close). 80 of these things are scattered around Pacific City. 25 in Hope Springs, 30 in Green Bay, 24 in Unity Heights, and a measly 1 in Agency Island.
These guys can be easy or infuriating. They will show up on your tracker normally so there isn't a need to press Up on the D-Pad. They appear as green "sun" symbols. The first few that you encounter can normally be captured by running straight after them, but later ones will require you to get a bit more creative. Usually, the best way to go about getting them is to chase them around and get a feel for their movement patterns. By doing this you can find a good spot to corner the orb and confuse it (it won't know which direction to go in).
There are 10 in Hope Springs, 10 in Green Bay, 9 in Unity Heights, and just 1 on the Agency Island. I know there is at least one orb in Unity Heights I found to be unobtainable while on foot (it is on top of the "Agency Tower Jr.," the second tallest building in the game). I actually just took a helicopter and hovered high above where it spawns. Then, I jumped out and fell on top of it. The speed at which you fall will be fast enough to prevent it from actually running away from you. The impact from the platform will kill you but you'll still unlock the orb.
Like the Agility Orbs, these will also show up on your tracker as you get near them. These will be indigo "sun" symbols on the tracker. In order to capture them you must be in a vehicle. Out of the 15 of them available the game recommends that you use the SUV for about three of them as they appear in off-road areas. Most of the time, you'll want to stick with the Supercar. Even for some of the off-road orbs, it is easier to take a chance and catching it right away with pure speed than to safely traverse the dirt (except for the one in the mid-left section of Green Bay's north island. Use the SUV for that one).
A total of 15 of these exist with 5 on each major island.
Hopefully this helps out a little bit. There isn't really too much outside help necessary to obtain all of the orbs in this game. Hell, the game practically just tells you where they all are anyway. Have fun and happy orb hunting.