Tactics for the Swamp (Fortress).

I prefer to rely not on wyverns, but on basilisks and gorgons. These are very strong warriors that can fight on equal footing even with level 7 units. Therefore, I only build hydras if I have extra resources. And if I can accumulate enough lizardmen, they can significantly reduce the enemy's unit population. The lizardmen stand under the protection of basilisks and gorgons, while the foolish enemies walk into the trap.

If the main hero is a sorceress, and there is a place on the map where she can learn magic of level 4-5, then the enemy doesn't stand a chance.