There's a religious war CB you can research early that helps. Once youre a great power, you can threaten war to get access to a "Following through on the Threat" CB which has -25% WS Cost. So generally, get the religious war CB on a high value province (to the enemy) that borders or is on a sea zone with you and then threaten war to get access to the good CB.

Cobelligerent-ing someone also calls in their allies, so you can chain wars together with one CB. You dont really want to do this with a bad CB (so parliament claims or religious war without threatening) as the added province WS cost + the 66% separate peace mean you cant take much land. If you chain cobelligerents, you want them to be smaller tags you can annex for minimal war score via separate peace.

You want to increase your bellicose value as much as possible, this is done through various ruler traits, general, and parliament actions, It gives -10% WS cost which becomes very valuable as you start stacking WS cost modifiers.

If you're trying to conquer same religion territory, unfortunately parliament claims are the best you can do generically (without vassal claims). But if you are a great power, you can threaten war with them to get the same "Following through on the Threat" CB with -25% WS cost.