I think the easiest way is to look for dynasties which already fulfill the conditions and then marrying one of the countries who can spread such a dynasty to you and then letting your ruler die without an heir. While you have a junior partner, you can't fall under a PU when your ruler dies without an heir.Playing as saxony, going fr the achievement; 50y in, so just started integrating Thuringia. Also just became emperor. My short term objective is to PU Poland with the mission. How do I most efficiently spread my dynasty? Marry to countries without an heir? My prestige is quite low because I had to do a disinherit, and atm only a few countries dont have an heir.
With diplo ideas, you could royal marry all heirless countries to which you can spread your dynasty(so no other country must be more eligible to spread their dynasty) and break the royal marriage as soon as they get an heir.I almost finished diplo though
Or you could go the other route and expand to 5 continents(or get the dynasty of a colonizer). You need to get to east asia anyway to get the Meissner Porcelain achievement