Prussia

Prussia is a minor power situated in Central Europe that appears at the start of the 1618 age. Later becoming a regional power during 1756 that can dominate the German region.

Overview
Even though Prussia is a small country at 1618 and is always risk of being pressured by the Polish-Lithuanian Commonwealth unless the player unites all Germany with this country. It boasts a decent economy capable of supporting 2 divisions instead of 1. Use it as your advantage to expand your influence on German lands. In 1756 it greatly expands, holding all Prussian, Brandenburg, East Frisia, and some Pomeranian lands and capable of quickly taking down other German countries except for Bavaria

Early Game (1618)
Try to ally with a major power (Especially Russia, France and/or England) by convincing them to ally you (must be a player nation). Even though you could expand your power without any help.

Be picky about your next target as you cannot hold nor commit a major attack. Usually Silesia/Pomerania is the best choice, but if they are allied to a strong country. Go invade Brandenburg as a last resort because they usually have no good allies and prepare to invade Saxony. Then invade the northern German States as you go expand your influence.

After all conquests on the interior German Lands. Declare on Denmark and PLC (If possible with allied help) to acess even more German Lands. But be careful not to expand further because there will be a coalition against you

Mid Game-Late Game onwards (1756+)
By this era. You need to ally superpowers during this era (Ex: Sweden, PLC (after the conquest), Denmark, Russia, Spain, Austria, France, Ottoman and UK) to help you manage coalitions against your expansion. Then invade the rest of Germany with the help of your allies. If you wish to expand even more, you could betray Austria and annex all German lands possible (All Austria + Sudetenland and Oberlausitz).