I'd say that holy-warring is still the most effective way, especially if you have the full force of an empire behind you. 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. You should be able to grab a few duchies at a time trough holly war, and may even get them without having a truce.
Great Holy Wars will allow you to take a whole kingdom at once, but they are much more difficult. Other Catholic are more likely to join the defender than during a regular holy war. Still, catholics kingdoms tends to be quite divided.
Then, you will need to convert them. The most efficient way is to give them to vassals, who will each have a court priest to proselyte inside the demesne. Unless you are going North Korea style, you'll have to give them away anyway.
Another method, much more difficult, is to try to capture an heir to a big chunk of land. Go pillage the castle where he is. if you grab them young, you should be able to convert them to the One True Faith™. Their future vassals will not be happy, but maybe you can try to help him once he is on the throne.
If you capture one more aged, force them into a marriage with a Norse. You can get your dynasty on a foreign throne, but it will not be easy and requires quite a bit of luck.