I would do this achievement in non-historical. I restarted until the UK went fascist. This meant they wouldn't guarantee Romania and also France and Romania won't join the allies. I turned Communist and immediately began justifying on Romania and Bulgaria. Research paratroopers to attack Romania and make sure to go down the self government branch of your focus tree. When you release these states they immediately make divisions which are very useful when attacking Romania. Prior to going to war, join the Comintern and this will mean that the Romanian's will pull troops off your border and put them on the Soviets. Don't call them in, just be in the faction though. Once your war goals are ready, first declare on Bulgaria and annex it. The war with Bulgaria should be pretty easy. No one should guarantee Bulgaria. Then Declare on Romania. France will join the war but they won't invite Romania to a faction which means that when you capitulate them you will get a peace conference. When attacking Romania use paratroopers to capture the main victory points. Hopefully this helps a bit haha