The player's country:
- is Corfu.
Core-fu is an achievement that requires the player to revoke five or more cores in a single peace deal as Corfu. One of the best targets to demand revoking cores from is Venice. Venice starts with non-owned cores on Zeta ( Serbia), Lezhë ( Albania) and Corfu, and the player has to fight them anyway in order to become independent. It is less difficult and more tedious to complete, as the player will likely have to restart if they cannot immediately find a strong nation to support their independence.
The achievement can be completed in just two wars. The first war, the war for independence, can be fought just a few month after game start. The player should be able to get their independence supported by Venice's rivals (e.g. Austria or Hungary). The goal of this war is independence and stealing at least two provinces from Venice. After waiting for the truce to run out the player can declare a second war (Austria will usually join if you promise them a province which you don't even have to give them since you will get the achievement after this war anyway). The goal of this war is simply to force Venice to revoke five of their cores.
If the Ottomans warn you right after gaining independence, don't worry. You can just declare war in the month the warning runs out (the game may show that the Ottomans will still join, which is just a display bug). The AI needs at least a month to send a new warning. Or, by taking Euboe and Durres in the first peace deal, simply ignore the warning altogether because Venice is no longer bordering the Ottomans.
Playing a legitimate Corfu game is actually not difficult if the player handles their situation with the Ottomans and Austria correctly. During the first war, it is important to take Verona whilst also making sure to maintain trust with Austria or Hungary. Even before Verona is fully cored, the player can force-move their capital by accepting the Byzantine nationalist's demands. Releasing Corfu back to Byzantium this way forces the capital to Verona, the common Exodus strategy for OPMs, and effectively brings you out of Ottoman gaze. The player can then improve relations with Austria (or whoever is the current Emperor) and join the HRE and perhaps a trade league for protection. Ferrara and Milan tend to be frequent allies, though conquering Mantua will likely not be worth the AE. Sometimes Milan will instead turn domineering and issue a warning, though admittedly it is less threatening than one from the Ottomans. Again, if the player has managed to ally Austria, the 2nd war with Venice will be easy, regardless, so long as you promise them land.
After Venice is dealt with, the future of the game is up to the player. Once AE has died down, Mantua or Ferrara are both viable targets since your force limit can be pushed to surpass theirs. If the Shadow Kingdom event fires, it's recommended the player stays in the HRE if they joined before. Both the loss of protection and the opinion penalty for leaving are not worth it.