As you attempt to leave Villachula for the first time, you are stopped by Uay Coco, the final skill trainer. In order to fully upgrade, you need to get kills with your chosen super move, pay 1500 gold to upgrade the damage, get 20 more kills, then pay 3500 gold.
There's two ways to actually one-shot. One is to go back to the early parts of the game and attack low-level enemies. However, it's also possible to earn this by hitting an enemy with the move and knocking them into a death zone.