Probably the easiest way of getting this achievement is to start off as Austria, then do something to improve relations with Bohemia (just send a gift or bestow imperial grace). The next month, you should be able to offer a royal marriage. The month after that, claim the throne, and then declare war as soon as you can. They won't have many allies (usually just their vassal and one other nation) so you should be able to handle it easily. You can even drag Saxony in by promising them land in the war, if they aren't allied to Bohemia. This is also a good start for an Austria run anyways.
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.