If you want to make life that bit easier for yourself, go for this achievement first...
Run round the city wiping out each and every Cel stronghold... thus cutting down the number of enemies you'll run into when you go chasing after the other objectives like Freak breaches and Absorption Units and Beacons...
To secure a tactical location/Cel stronghold, locate it on the map, head to the location, and hit the 'back' button to call in the helicopter backup... You'll be told that there's too many Cel in the area and a meter will appear in the top right of your HUD so you can see how many you have to kill before your reinforcements will arrive. On your radar, you'll see all the relevant Cel operatives. Ordinary Cel members won't count towards clearing a stronghold - only the ones on your radar.
There were three strongholds that I found a little more difficult... One atop the buildings just east of the fairground - a lot of grenade launchers meant that you were being picked up quite quickly and juggled for a while... another tricky one was in Unity Heights - mostly down to the raw firepower they were laying down (a lot of heavily armoured guys wielding turret launchers and gatling guns, and a lot of other explosives toting Cel guys)... and the trickiest one was at the plaza in Unity Heights, partly because I'd cleared out all the surrounding Cel and was only a fraction down the meter, though I could see plenty of them on the radar... Hint... They're all high above you! Not sure if there was an easy way to climb up to the level where they were, but I went and picked up a helicopter from the agency tower and once I'd cleared out the guys on ground level, just flew up and toasted the rest of them from the safety of my helicopter!
Hirsute Dave points out the rocket boots from the Toy Box DLC will help out greatly with this... Great advice, and thanks for reminding me about that (don't think I had the DLC when I wrote the solution initially!)... They also help out with doing the wingsuit missions as well....
Note that some of the locations are linked, and if you don't complete all the linked locations quickly enough, Cel will overrun the ones you've already completed and you'll have to do them again!