I found the easiest way to get this achievement is to play as Austria and get 7 of the HRE reforms, which should leave the entire HRE as your vassals (without any of them using up a diplomatic relations slot). Using this to your advantage, conquer from all the nations that surround you and never get the last reform until you get this achievement.
If you have any ideas on how I can improve this solution, I'd love to hear them! There may be something that isn't here that should be, or an easier way of doing this. I'm always open to suggestions. If you notice any typos or poorly phrased sentences, please let me know.