To fully upgrade the Leviathan Axe, you need Frozen Flames, which is a type of crafting component. Level 6 is the maximum for the axe. The first upgrade is given automatically when you meet Brok for the first time. You will then keep getting Frozen Flames from story bosses. Even if you fail to collect them, they will be added to the "Lost Items" section in the shop. By completing the story, you can get the axe to Level 5. Getting it to Level 6 is hidden in the shop under "Buy" - "Resources". You can exchange the Chilling Mist Of Niflheim for the final Frozen Flame. The Chilling Mist Of Niflheim is found in a chest in the central chamber of the Niflheim realm, which costs 5,000 Mist Echoes to open. You must do Sindri's first favor in Nifleheim to gain access to the central chamber. Then, keep going around the "labyrinth" in this region to farm Mist Echoes until you have 5,000. Once you have enough Mist Echoes, open the chest to get the Chilling Mist Of Niflheim. Return to a shop and exchange the Chilling Mist Of Niflheim for the Frozen Flame. You can then purchase the final upgrade for the Leviathan Axe and get the "Worthy" trophy.