Start as Argentina, and rush down the Fascist path by doing the following focuses:
- Roberto Maria Ortiz
- Work With The Nationalists
- Cut Ties With Britain
- Support Radical Nationalism
You can use the Fascist advisor after the first focus to get your support to 60% as well. After these we get the extra research slot by doing:
-Corporatism
- Reestablish Ligas Patrioticas
-Fascist Researchers
Now we take Uraguay by doing Establishing a Foothold and declaring war. You should have 60% by now, so flip fascist by:
-Italo-Argentine Diplomacy
- Union Nacional Fascista
From here you can continue going down the political tree, or working on the economy. You will need to build up a bit of an army before you attack Brazil, who you can manually justify on or use the focus The Old Enemy to get a war goal on. Once Brazil is taken care of, use the navy you stole from them to get supremacy across the ocean and justify on Spain! When ready, win the war and the take all states for the achievement!