I usually get an alliance with either France or Russia as first priority to beat up up Austria and grab the sphere of influence. Defeating France as Prussia is a tough fight, but certainly not impossible if you beeline some mil techs. The Prussian army is really good. You mainly just want Russia on side for that war to keep them off your back, not actually to help with the fighting.
If you can get soldiers to all of the sectors around an enemy, then defeat them in battle, they instantly get destroyed instead of retreating. Encirclement is a viable strategy for battle, then. Make sure you mobilize all your reserves quicksmart and get them into large fighting units tho.
If you want a real challenge as Prussia, you can try to form "Great Germany" by sphering Austria before you form Germany. If they are in your sphere, they get folded into Germany just like all the other Germanic minors. The trick is you've got to beat them down out of Great Power status. Usually by occupying the whole nation until their factories all go bankrupt and they fall out of the great power game. Gotta act fast before they bounce back though, so everything else has to be in position!
