Ever since Art of War the new Papal system has rendered this achievement unachievable.
That one is easy. Start as some1 in HRE (Austria is the easiest option). Make an alliance with let's say Saxony. Dow on biggest alliance inside HRE (5-6 minors). Beat them all and after that transfer occupation of province to Brandenburg , sign peace with all OPM seperately (annexing 5-6 OPM in the name of Brandenburg). Leave alliance with Brandenburg and join bandwagon coallition against Brandenburg.if you think thats unachieveable then try the one where you have to be in a coalation with 5 countries.
( its even harder than the great khan for me .-.)