Man that took me a while to find too. I was like "I wish there was some wiki page where I could just read about it and see helpful screenshots." Then I wrote Zona Tres Upload.
Balística Burglary took me even more time. I had blown up like half the base, trying to find something to activate the doors. Then I noticed the hole in the ground a little further away. It was pretty annoying to know that there's no page for me to get directions from. You're welcome, by the way.
But I wouldn't call these "hard". I suppose someone who might try to kill every enemy and insist on never using cover, would find Balística Burglary really difficult, but normally the hardest missions are the ones where I have to protect a transmitter. I normally have to restart from a checkpoint with these.
Trouble with helicopters is that they are very hard to keep stable enough to actually fight another helicopter with in JC4. This makes fighting other helicopters nearly impossible in JC4 because the a.i. doesn't have that problem and nearly never misses. I suppose in a mission like this the enemy helicopters might be giving more attention to the transmitters than to Rico, so it would be easier than usual.
To really efficiently defeat helicopters, the player has to grapple them to the ground. But in this mission the transmitters are so high that the grappler can't reach anything.
So in this mission the most efficient thing to do is to grrapple to the helicopters and hijack them. Then just immediately abandon the helicopter, letting it fall to the ground. And then repeat this with the rest of them.
I got bored and dropped a cargo plane on the transmitter. Somehow it didn't do any damage to the actual transmitter but it was big enough so that it messed with the drones and helis. They just flew into it. I took out the stragglers with the AA gun.
Story-wise I know that the garland missions are suppose to finish before the south-east is taken over. And by the time of the last mission Oscar mentions that the AoC controls the "majority" of the nation. I know I took over some areas that I didn't have to... so I don't know if the minimum really is 51%. The minimum before The Illapa Project is probably about 60%... The actual minimum might be lower because some areas must be captured to unlocks missions, or to capture areas further away that must be captured, but there's a few that probably don't have to be captured, but which make sense to capture anyway.