Here is the answer::)
1)You need to approach the first tent with an army of
20 peasants; 10 imps; 1 gremlin; 5 elves; in this exact ratio... but in other cases only such
2) You need to approach the second tent with an army of
25 zombies; 20 vampires; 16 liches;
3) You need to approach the third key-holder with an army consisting of
2 devils; 2 bone dragons; 1 colossus; 5 angels;
Well, that's all, good luck!
P.S. You must approach the key-holder only with this army; others should either be dismissed or left to defend the mine so you don't have to dismiss them, just in case someone comes in handy. After completing the third garrison, there is a ship—get on it and the mission is won. Glorious victories to all!:)