Still, catholics kingdoms tends to be quite divided. Other Catholic are more likely to join the defender than during a regular holy war. Great Holy Wars will allow you to take a whole kingdom at once, but they are much more difficult. You should be able to grab a few duchies at a time trough holly war, and may even get them without having a truce. If you attack at a time where your enemy is divided (succession crisis, during another war etc.) they may not be able to call allies, while you have the full force of an empire with you.
I'd say that holy-warring is still the most effective way, especially if you have the full force of an empire behind you.
