Other players have mentioned waiting for an Empire to implode so they can be picked to pieces. How long do you actually wait for things like that to happen- 100 years?
It is easy to see. Start a game, and from time to time, take a look at Byzantium and the HRE.
Because of their bad starting monarchs, lots of dukes will rebel.
When you see that their king is bad, they are in numerous wars with former vassals, and lots of dukes are not part of their kingdom, you can try your luck.
Sometimes they survive though! And a HRE with personal demense provinces all over the place (1 in Italy, 1 in Prussia, 1 in the holy land, 1 near france) is hard to take down with a small army.