I've been having a go at this, an interesting tactic that can also help if you're Catholic or often kill lots of catholics;

Raid an enemy settlement, sack it, sell all the buildings, get your troop out, leaving one token city defender unit and wait till the enemy try to get it back. Now sell the settlement cheaply to the Pope for 3-6k.

You get a huge reputation boost from the pope, the attacking faction looses reputation for being on Papal soil and will often blindly attempt to attack the settlement anyway getting themselves excommunicated. Quite often the settlement will rebel anyway as the Pope can't reinforce the place fast enough. I suckered Milan and Sicily into this. I've sold the pope 4 regions now and all of them either fell to rebels or to the faction I stole it off.

Despite killing off the Spanish, the Portugeese, Sicillians, most of the French and Millaneese and despite being an Islamic country I've still got better relations with the Pope that ever did as the English.