If you are going to play with Prussian in either Armageddon or the Abyss, you must keep in mind 3 things:
|. You are going to be the battlefield of 5 nations.( Rome, Swedden, "France", Soviets and Russia. The cossacks don't use to DOW)
2. It going to be very frustrating. (For something no one had try to write a guide for this scenario with Prussia)
3. The "historic description" is right the opposite of what you are. You are essentially screw in middle of 5 possible enemies.
Personally I recommend the Abyss scenario, cause it give you a little more time, and low aggressive settings (But even with this, the peace isn't going to last more than a few months anyway).
You start only with the more elemental things, and your research teams left much to desire (At least in comparison with the level of the techs). The plus side: You can research whatever you want. One of the first techs that you should try to archive is Infantry '36, trankette and any land doctrine.
Keep in mind that in land doctrines the IA prefer the Firepower path, but the Mobility path gives you bigger early advantages. Also, seems that the IA ignore all the naval aspect of the game except for the initial naval force.
You don't have enough resources to run your industry even to begin with (All are in the same position). You should immediately attempt to trade as much supplies as you can to get a bare minimum of war materials flowing in. Unfortunately everybody else is in pretty much the same position as you, so don't hold your breath. Don't even think about building (factories, AA, coastal fortress, infraestructure): It will take a long time before you could capitalize any possible benefits, time that you don't have. Use all you IC in recruiting soldiers, cavalry and tanks(This ones in less proportion cause it huge time to produce).
You don't really need to worry about this: Your possible allies will do it for you. And even if not, they will send you request to joint to axis (and get war with half of the word in the process). If you want, try to raise the relationships with Sweden, Italy or France and then reclaim your territories from them (Works surprisingly quick if they don't DOW you before)
I recommend move your troops immediately to the west and DOW the European Soviets. Why? The democracies don't use to DOW early, so your Cossack and Swedish frontier is safe for the time being. After that grave all the land that you can before their troops reach the coast, and crush them as they come. In my case, their left Great Britain almost unprotected: If is the case attempt a amphibious assault with a couple or two of units.
At some point, the Comintern will be formed and Rome will DOW and invade you. Don't panic. Use your planes to gain time (and make a good damage), and finish you first front before move your troops. The ottomans can help.
In this part I use freedom to move all my sliders to the positions that I wont. Anyway, it isn't much difference cause your troops take a eternity to be ready and you can really much in the incoming troops. I recommend go for full Central Planning first, but it just an opinion.
No AI player will ever use the intelligence system, so if you want to ignore it go ahead. It can be worthwhile to build up spies to gain info about the other country's army, but unless you want to spend a lot of time and money on other operations, it's probably not worth it.
Try to form the biggest and faster army that you can. The only plausible form of victory is destroy all the possible armies the faster that you can. Surround your enemy, leave it without supplies and crush them. Also, attempt to destroy their Air forces capturing their airports.
Try to keep a maximum of 2 fronts at the same time. In other words: Try to keep a friendly France while you beat the soviets and delay all what you can the entrance of Rome into war. Unfortunately, the NAPs are useless (They will attack someone of you alliance and you will take 5 of dissent for defend it).
The democracies won't work together (so you can expect some peace offerings from them), but the commies and axis will do.
Expect a concentrate of air power over you when you have the upper hand in the war, so try to capture quickly the airports with airplanes.
I didn't finish this scenario cause was to frustrating for my taste (a bug that changed my available IC day to day in the form of a variable "peace time modifier", even when I was at war almost right from the beginning; the multiples fronts; the crappy armies; etc) , but I as able to annex the Soviets, Sweden and Rome. Took it a while though.