Advice with diplomacy; form letters perhaps?

[Dedas]

Although that could work it is very risky, especially if you want to focus your energy on another foe. The opponent you threatened into peace could suddenly swing about and stab you in the back, or even ally with your enemy to coordinate the thing. Threats between equals almost always lead to a weak peace. Your are better of agreeing (at least in principle). Later on, when you are stronger than him, you declare war or stab him in the back if you feel deceitful.

Always hide your true intention (if you are not roleplaying a honest knight type or similar).

[Gregstrom]

If the opponent has allies, you could perhaps deal with them also. Get them to make overtures on your behalf.

[Digress]

If the opponent has allies, you could perhaps deal with them also. Get them to make overtures on your behalf.

Be careful with that one - sometimes the only thing it results in is their ally sending them a message saying "Lets divide this players empire as follows - I will take provinces A, B and C and you can have the rest ..."