Magister Ladd:
You still haven't found Blood Fire, have you? In order to create the elixir, you need to melt the two ingredients fast and perfectly. Normal fires are not hot enough to achieve that. Only Blood fire is hot enough to meld true gold and pure silver.
Go kill the bloody guardian in the Lair of Antharas to obtain the Blood fire! Then come to me and I shall instruct you how to create Mimir's Elixir.