I have won a few Crusade defenses and it's not easy. I was just typing about this, hope I don't sound like I'm repeating everything in all threads here.

As Lithuania last game I defended against three Crusades, I had roughly "80%" on my side, Catholocism was mid 150s%.

In my experience they tend to attack in many small armies, but as long as you can put together a single army to defeat their larger stacks (I saw 15-20K as bigger stacks) you can defeat them. You're warscore is going to go wayyy down for the first few years because you can't stop them from besieging your territory, but as long as you can defeat their armies and assault what they've taken (should be low garrisons for next to zero loss on the assault) you can hopefully stay above water. I was down to 60-80% LOSING all three times before I turned it around, and then it's a matter of keeping them off your territory for a while. By like year 8-9 I was seeing 30% warscore swings for the occupation of a single province, so it becomes crucial to be vigilant in keeping them out.

It took 10 years, 10 years, and 12 years. These wars take forever, do not expect to win quickly.