Hmm, let's think. Where should I start.

Playing as spain I used several nasty methods.

First of all I killed 8 popes in 25 turns:D, just make sure you have a 10 subterfuge assasin and you have about 30-45% chance, depending on the Pope.

Further on, after getting a warning of the pope to stop hostilities I executed the Portugues King, and 800 of his man, which I took prisoner in a very strange battle. Although it cost me 17000 in the turn itself bij not ransoming them, I think the worth of getting rid of the army was more than worth it.

And not to forget, the absolute massacre of the Moors. Destroying and killing everyone and everything in each moorish settlement you take sure get's them down on their knees quickly