Defeat the final boss (part of the story). Once defeated, after the credits, you'll return to an autosave made right after the fight

Head back to Hunter's Settlement and talk to the Dragon in the endgame (just before fighting the final boss)

In the endgame, defeat Malik at Hunter's Settlement

In the endgame, go to this hidden map and defeat Volgus
You'll need to use the item "Game Breaker", which you can buy at the Cosmos Shop

If you've already used that item, check out this discussion --> The iron giant Superboss (SOLVED)