So after hearing how OP Prussia is these days I decided to give it a go myself. No vassal core feeding for avoiding ADM cost or anything. Purely military playthrough with a total of 3 diplo vassalizations of 2 province nations (Walachia, Styria and Ulm) and 0 PU's. Everything was either conquered or force-vassalised. No religion idea for the +100% opinion regeneration as some people say is "vital" to conquest oriented nations. As you can see I used none of the things people say are the only way to avoid a massively annoying game where you can hardly grow because of huge coalitions.
Funniest fact: Teutonic order survived, they would've been a lot bigger had it not been for me wanting their land all the time. They ate through Poland (but did not destroy them) and ended up in the Ukraine. It was a shame that they always had land I wanted or I would have nurtured them.
My ideas:
I took every single military idea in the game to make my army as strong as possible. I took the innovative idea to keep my army tradition at a steady 100 and I took economic ideas because at the start of the game I kept getting inflation event after inflation event which added upo to 15 inflation. I just had to get rid of it and economy would allow my tax income to get even higher which meant a bigger army. At the end I took trade just for the hell of it because I didn't really need anything else.
My nation:
Everything people complain about happened, Austria inherited Burgundy within the first few years of the game and blobbed like mad in the HRE. Bohemia came close to enacting the ewiger langfriede but I managed to become emperor just before that and held on to it. The entire south of my nation I had to conquer from Austria bit by bit. France was my ally for most of the game except for one time where I ended up in a war against pretty much every single great power in the game and destroyed their armies completely and utterly. Nothing could stand against my armies, I just walked them around the map and anything they touched was completely destroyed, more often than not even in a single battle.
My army was the largest in the world and I could've easily made an army even twice that size but I didn't really see the point in that, I didn't need more troops at all. Didn't become an absolute monarchy untill the end of the game which was pretty stupid, had forgotten all about it.
Now this is all end-game, the beginning of the game was far from easy. I had lots and lots of setbacks, I lost some wars, incredibly high inflation, times where I had 0 allies due to them losing wars and being forced to abandon me and strong alliances in the places where I wanted to expand. But in the end I made it, and how.
The only time I had a fairly decent sized coalition against me was near the end of the game when I wanted to tidy up the borders and took some more land than I had originally planned to. That ended in 1 fairly big sized coalition war which I of course easily won.