Normally I'd agree with you about the blobbing easiness but... how did you do that as a Catholic? You could invade the HRE I guess, but that still leaves conquering France and Britain. Was it the "grant a claimant a county and then press their claim" trick?
I'm not sure about how he did it, but having the pope as a vassal is an extremely powerful tool. You can ask him to give you any claim, and he can't refuse (when you're on the screen of a title, with the buttons destroy, claimants, etc, there is a claim button, that sends a demand to the pope to give you a claim on that title, costing piety).
To get the pope as a permanent Vassal:
Step 1: See if you have any of your bishops are cardinals. For any of this to work, you MUST be an emperor. If no bishops of yours are cardinals, see step 1a, if you do have some, see step 2
Step 1a: Create a YOUNG antipope
Step 1b: Press claim on papacy, the pope is now your vassal.
Step 2: Put lots of money into the election fund for cardinals, until your nominee is winning by a long shot
Step 3: Assasinate a cardinal
Step 4: See step 2 until all the cardinals are your bishops. If you created an antipope and pressed his claim, congratulations! Keep that campaign fund high, and you will never lose the your new vassal. If not, see step 4a:
Step 4a: Assasinate the pope.
Step 5: Congratulations! Keep that campaign fund high, and you will never lose your new vassal.
Edit:
Step 6 (for super easy mode): Increase church taxes to max, and church levies to minimum. Bribe/whatever the pope to positive relations, and tada, money forever!