GremL!n
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I personally see two ways to complete this mission.
1. Rush, which has already been described. In general, this is the most correct way, but if it doesn't work, you can try...
2. A long and exhausting strategy against the AI. We wait for 1 week, buy everything we can in our city, quickly capture the first Elven city and start building it (it's better to build Ents right away). We wait and accumulate strength. Then, a hero with superior forces will attack us. We surrender the city to him without a fight and go to another Elven city. Everything will already be built there, we buy everything there. They will come after us again, and we surrender the city again and go to the third. And so on in a circle. The AI will divide its army into parts, and sooner or later we will defeat first one half of its army and then the other. And AS QUICKLY AS POSSIBLE, KILL THE DRUIDS! They create neutral creatures on the map that join the AI's army.
I just completed it using the second method, but it's long and tedious, and requires experience. Moreover, it's so long that I received 72k experience for the final battle. If there wasn't a limit....
P.S. I'm curious, on what difficulty level did everyone complete this mission? I did it on Veteran.