Easiest way to do this is to, as soon as you get enough political power, start justifying a war goal on Poland. So long as this is the first war goal of the game, Britain and, to a lesser extent, France are unlikely to guarantee the Poles. As soon as you have your war goal (should be early-mid 1937), declare war on Poland and start justifying on Germany. Due to the world tension increase in the last few years, including your recent invasion, the justification on Germany should be done by 1938 and, as the Germans are both a major power and a fascist nation, nobody will guarantee them (except Italy and Japan, assuming they complete the Tripartite Pact before you attack them). The most important part of the achievement - make sure you don't capitulate Poland until you attack Germany. This means, for you, that Poland should hold Warsaw, Krakow and Danzig until you start attacking Germany. Whilst taking any of these cities doesn't necessarily mean they'll capitulate, these are the biggest VPs Poland has at the start, and taking all three is practically guaranteed to finish them, especially since they're all in the west and you're attacking from the east.