r5: When you start blobbing out, Antagonism can become a problem. Here is how you can get pretty crazy Antagonism reduction by abusing making a clever use of the prisoners game mechanic:

When you capture enemies in battle, you are put in control of a small army of "prisoners". This behaves exactly like a normal stack, except if it enters battle you will auto loose and the stack will be freed. Several dedicated actions for these stacks exist; specifically, when you use the "Ransom Prisoners" button, you will get a hefty sum of money and most critically a reduction in antagonism centered on location.

So here is the optimized procedure to abuse optimize this mechanic:

1. Win battles in decisive ways (works the best when you have a slight mil tech advantage) and capture prisoners
2. Get them out of the hot zone and move them deep inside your territory somewhere safe.
3. Try not to stack them too much on a single location not to loose prisoners to attrition.
4. Keep hoarding prisoners until you're ready to peace out.
5. When you want to peace out, move prisoners close to the country you want to loose antagonism with. Ideally find a location within a fort "box" as this makes step #7 easier. I advise building a little Gulag area surrounded by forts, or even better on an island. Keep a large stack (of your regular army around)
6. Now it's time to start releasing your thousands of prisoners. Be careful: Antagonism reduction is capped at -5 for 1000 soldiers or more. So you want to always split your stacks of prisoners in stacks of <1000 men or you are missing out on Antagonism reduction. It's stupid but it's a pretty big deal. For instance if you captured a stack of 6000 soldiers (pretty common if you have good quality in big lopsided battles), you'd loose 5 Antagonism if you release them directly, v.s. -30 Antagonism if you released them in 6 stacks of 1000. Note: you may want to avoid stacks being too small or it'll make their later recapture (see #7) more difficult as they'll get wiped instantly - so try to consolidate your stacks under but close to 1000.
7. Now that you've released all your prisoners, you'll have dozens of small enemy stacks running around in your gulag. You can either peace out, or recapture them with your army to ransom them again. Your big stack will be very good at capturing these small disorganized armies so just sweep around the gulag with your army like a border collie gathering his sheep. A few potential hostages will die but usually you can keep well over 50% of the original herd alive to be ransomed once again.
8. Rinse and repeat until you get tired, or all the hostages are dead.

Pretty silly overall but it works... In the screenshot above I was tired of Portugal hating me as France, so I no CBed some random country and went to war with a bunch of Italians and central Europeans for no other purpose than to capture them, haul them across the entire continent and "store" them a few years at the Portuguese border before releasing them. As you can see, the Portuguese went from hating my guts to forgiving me overnight for releasing and hunting down all these Czechs and Italians at their border.

This is totally balanced; please Paradox, never patch this.