Just Cause

You must complete 10 side missions.

I suggest leaving the Side Missions until the end of the game after you have taken over all the settlements (both Military and Montano, but isn't necessary) as long as you can unlock enough guerrilla points to go up in rank to unlock a helicopter that has missiles on it.

After you have obtained a decent chopper, unlock Guerrilla Base #23 (the most southern extraction point) by taking over the military rural area just to the South of it.

Now head south from there to the Military Base, its the only one on that little southern island, its called Los Islas Defence HQ. Take over that base, check this solution by lozzy7 for tips if you are having trouble:

Now you can start grinding them, head back to the #23 via Extraction, jump in your chopper then head back to the base. Talk with the Guerrilla side mission guy (will have green arrows pointing down on him, or a green "!" on the map).

He will give you a side mission and it will NEVER have you go past the little island, he will always send you north about 800M and do something (kill a car, chopper, people, trade item, etc.) The reason you want the chopper with missiles is so you can kill car/ choppers quickly and without a hitch and not have to chase them around.

Now just keep completing them, you should be able to do one about every 1-3 minutes this way. If your chopper explodes, extract back to Base #23, and if you die you will automatically start at that #23 as its the closest base.

Keep grinding and this will eventually be yours!