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To reach Navea City, simply go to the nearest Warp Portal and then click the Crystal. Now choose Navea City in the list to teleport to that location. You can purchase the Fusion Formula from the Gunslinger Master, Turin. He's in Navea, found at the upper-right corner of the city along with the other class Masters. You can also use your auto-navigate by pressing M and then search for Master Gunslinger Turin. Click the boot found at the right side of the NPC tab to make your character move and navigate by its own. Purchase the Formula from Master Turin using your token. Tokens are obtained from Daily Quest rewards, so make sure to do your daily quests every day since these are found useful for item exchange. The Turin's Executioner Fusion Formula costs 100 tokens.
Head to Navea. Look for Master Turin. Purchase the Fusion Formula.