Hail the Alliz Tae summoner NPC once all the protectors of fear have been killed, it will ignore you but two in-game messages will appear in your chat box indicating that Alizasaur is spawning. Alizasaur will spawn underwater at ( -974, -17, -16 ) Copy/waypoint -974, -17, -16, make his way to the summoner NPC, kill him, and then return to his spawn location.
His treasure chest will drop right in the middle of him. Just right-click his corpse and select 'loot' to remove his corpse from in-game and make his loot accessible.