Hearts of Iron IV

Starting as Spain, go down the Republican branch and, as soon as the civil war kicks off, you'll get an event forcing you into one of three paths. Choose the one that unlocks "Regional Defence Council of Aragon." This starts a timer until the anarchist uprising. Hold the line against the nationalists and their German and Italian volunteers (optionally, bring in your own Soviet volunteers, but remember that these will be enemies once you flip to the anarchists) until either this timer is up or you take the "Masters of Our Own Fate" focus. This will turn the civil war into a three- or four- way, depending on whether the Nationalists chose Franco or either the Carlists or the National Syndicalists to lead them (Franco, unless something goes wrong for the nationalists, will generally avoid a split, whilst Carlists and Nat-Syns will always break free from each other). From this point on, the objective is simple - win at any cost.

To make this simpler on yourself, rush "The Maximum Concession" and "Seize the Gold Reserves," then ignore "Masters of Our Own Fate" until there's only seventy days left until the anarchists break away. This will ensure you still get the bonus attack and defence against the Republicans whilst giving you time to work on your other focuses. Also, make sure to get "Foreign Arms Purchases" as soon as possible for an extra 1000 infantry equipment every few days, providing an infinite source of weapons for as long as the civil war drags on. Finally, though this is only if you want to play past the achievement, make sure to get "Portuguese Anarchism" before the civil war is over. That way, you won't have to rebuild your military before attacking Portugal and, when the Portuguese anarchists win their civil war, you immediately annex all Portuguese territory. If Portugal cores all her colonial territories, you will also gain cores on these when you form the Regional Defence Council of Iberia.