Hey, would anyone have any advice on taking land from the Holy Roman Empire?
I will assume you are smaller than them and catholic, which allows for an Invasion to be requested.
If you are roughly the size of France: just wait for the HRE to enter a difficult war, then request an Invasion and declare war on them. Just mass draft and gather your troops in stacks of up to 10,000 men, then hit the Emperor's personal holdings full force until your war score is at 100%. The odd part is that annexing a large Kingdom/Empire is often little harder than annexing their smaller counterparts, since the war score gained from occupied counties isn't really proportional to the nation's total overall size.
If you are roughly the size of Aquitaine (the kingdom) or smaller: do the same as above, but try to save 900+ ducats for mercenaries before declaring war; conquering land at an accelerated pace is essential as, if the war drags on, the HRE will unavoidably stampede over you. Though the HRE's size is its strength, it is also its weakness, as once it becomes occupied with a front it has a hard time defending its more distant borders.
While this is an achievement most easily accomplished with France, you can also start as William of Normandie and conquer England, France and the HRE, in that order; France is larger than England and the HRE is larger than France and England together, allowing for Invasions to be abused without a proper claim.