pounded out = they worked very hard (very aggressively) to reach an agreement.

Thank you very much c2check!
Can I use it as well for someone who works long hours and is very prolific? Or is it limited to aggressive interaction between two or more parties?

Here, aggressive isn't bad (not fighting between people).

You could say, "I pounded out my homework." That would simply mean you worked very hard and diligently to get it done.
Or "I pounded out the presentation that was due," means I worked hard, almost like a machine, to finish it.

Usually it is said when something is done in a relatively short period of time. (Like if it is due tomorrow. You work hard/diligently to get it done.)
You could also say "crank out".