My evil Nature Mage (named Erilius) resides in a tropical forest where he has abundant access to local life (of which all the flora and fauna except people can be controlled by him) and where he has survived for 10,000 years through a combination of magic and his innate ability to control the minds of the sentient native cat people (to be known in the future as Felinus).
Note that Erilius is not human and is the last of a formerly benevolent humanoid race of sentient magic users who helped humanity by solving our lack of food problem. After humanity and the elves betrayed him and drove his race to extinction he fled across a great desert and built his tower in the Rainforest on the southern side of it. He vowed to one day get revenge and drive us all to extinction and has been keenly studying hundreds of thousands of scrolls and hoarding the knowledge of magic his people had achieved to keep it from humanity for the last 9,980 years of his 10,000 year life.
He has finally succeeded in uniting the too numerous to count competing tribes of the Felinus and has amassed an army of 3 million warriors (he united the entire race under his banner). He invaded the vast but disunified southern provinces of the human kingdom of Erial and has totally destroyed the Royal Mage Corps headquarters in the southern provinces.
The King (John the Wise) has amassed an army of 50,000 knights, 30,000 archers and 120,000 disorganized regular infantry armed with halberds, all types of sword, spears and pikes and 20 of the kingdoms most powerful mages (still weak compared to Erilius) and set out to meet him in battle on a great plain surrounded by hilly forest where the knights’ charge will be the most devastating and archers, infantry and siege equipment will protect the flanks. These soldiers are the combined allied forces of his kingdom and two other more northern human kingdoms. The elves, a magic-prone race are hesitant to get involved because they naively believe that Erilius is scared of them.
If the Elves (with an army of magic-wielding warriors combined forces with the human Mage Corps and the human armies they might be able to stand against this massive Felinus army and Erilius for long enough to evacuate their peoples to the mountains.
What might be an effective way to reveal the light or the elves and convince them to join forces?