Playing as England, I forged a heretical alliance with Spain and Venice, the other two excommunicated factions.
The French Pope (who had the title of "the Corrupt") was making increasingly heavy demands, culminating with ordering me to send my faction heir on the Crusade. Well, I complied, and siezed Jerusalem with a force consisting of two units of peasants, two units of Mailed Knights, and a couple units of mercenary ranged cavalry. I promptly sent them back to their ships and started the voyage home, while also gifting the city to the Milanese, one of my more annoying enemies. French fleets sank my ships on their return voyage, killing my faction heir.
I negotiated alliances with the Venetians and Spanish, launching a united offensive against France. The Pope commanded me to cease hostilities, I ignored him, and I reduced France to Paris and a few southern regions, while also assassinating every member of their royal family and exterminating the population of every city and castle I siezed. At about this time, my finest killer-for-hire arrived in Rome, successfully killing the Pope. A Milanese cardinal was elevated, and died mysteriously half a year later. This proceeded for quite a while, until a Hungarian Pope was elected, Hungary being one of the few still-neutral Catholic nations.
All in all, I assassinated the entire French royal family and 4 Popes, while also capturing most of their territory, in 2.5 years.