I had 4 goes in total before I got it. I have not really played in a few months though, so I am very rusty
Attempt #1 - I lost the war as I didn't play well.
Attempt #2 - I missed out on the declaration of war as Bohemia got an heir a few days before I could declare it.
Attempt #3 - I lost again, but I think I was a bit unlucky in having my sieges take ages, and AI sieges finishing fast.
Attempt #4 - I got the union by July 1453
The basic strategy that seemed to work well is:
1. Delete ships they are kind of pointless here.
2. Set monarch focus to Military, for some reason it starts as admin despite your ruler being a 6-2-2.
3. Hire a mil adviser, preferably 5% discipline.
4. Get both stacks to size 12, with a general on at least one of them (maybe both but it will slow down mil tech 4. Mil tech 4 gives you a big advantage over enemies still on 3). The nobility have too much influence so just hire one or two using points. You start with 4 cav so go all inf. Add at least 1-2 mercs in each stack.
5. Camp your armies out near Tirol.
6. Use the nobility and clergy actions to get some points, should be able to get 100 mil points from nobility, and 50 admin from the clergy. Save the burghers until after 1450 so you can develop cheaper (I was developing Tirol for the gold mine).
7. RM Bohemia, Hungary, Castille.
8. Ally Hungary, Castille, but not Bohemia.
9. I did not lower any maintenance as the war can start quite early.
10. When Bohemia has a von Habsurg on the throne, then claim throne.
11. On the next month, declare war.
12. If Bohemia ally Poland it may not work, but for anyone else, should be OK.
13. Rush down everyone except for Bohemia. You may lose Wien to a siege, but you want to get it down to you vs Bohemia as fast as possible. Peace out smaller nations for cash/war reps.
14. My Bohemia was allied to Ferrara and Ljuneberg, I didn't see them ally Poland in any of my attempts.
15. By the end of the war I'd not only won it, but I had also defeated a couple of stacks of noble rebels, and some heretics. I had 2-3 loans, but that is a small price to pay for a union over Bohemia.