You absolutely need Hardline. Even the slightest bit of assistance helps.
With that said, run FOUR scorestreaks. Getting a 20 gun streak in this game is mind-boggling hard. Getting a 20 streak with scorestreaks is not that difficult.
Play Domination. Or Ground War Domination.
Scorestreak 1: UAV with extra assists and extra time. With Hardline, this shouldn't be that hard to achieve, especially after capping the initial flag.
Scorestreak 2: Assault Drone with Rockets. The key to earning a 20 streak with streaks is by finding an extremely safe spot to hide your body if you plan on using streaks that take you out of body. If you are a master of doing this, stick with just the rockets module. If you are not, run the rockets module AND the AI controlled module.
Scorestreak 3: Vulcan with extra burst, extra burst and extra time. That gives you THREE bursts, all with extended time. In Ground War, depending on the map, you can get up to 2-5 kills a burst. That's 6 to 15 kills in a single streak. The Vulcan out of the care package only gives ONE burst, and hardly anyone runs the Vulcan otherwise, so no one is expecting a second burst, much less a third one. They will run out into the open after the first one, thinking the coast is clear.
Scorestreak 4: Orbital care package with double tap, support and extra odds. You'll get another streak for "free" basically -- if you're lucky, you'll get the suped up Warbird/Paladin.
So the key to this loadout is to first earn all your streaks. Don't call them in separately. Wait until you get the orbital care package, see what you get out of it, and then call them all in. Find a VERY peculiar spot (such as up on a ledge in that enclosed room in Solar -- or in a patch of grass that almost 100% camoflauges you on Greenband).
If you manually control your Assault Drone, you can probably get 5 to 10 kills. Then call in the Vulcan for 6 to 15 kills. Then call in whatever you got from your care package (or if it's AI controlled, like the Warbird or the assault drone, call this in first before you do your own earned assault drone).
The UAV aspect will just aid you in getting these other streaks faster because of the "extra time" and "extra assists". If you are a skilled enough player, you can also add the "scrambler" module -- or take off one of the other two aforementioned modules. Using the scrambler before you call in all your streaks is IMMENSELY helpful, as people won't know where you are attacking them from, especially with the Assault Drone and the Vulcan.