Tactics for the Swamp (Fortress).

Well, Tazar will be stronger than Bron.
I didn't understand the part about switching. Wyverns are built after the lizard lair is constructed, meaning at the earliest on the second turn, if there are enough resources. The creatures may not be very strong in combat, but they are quite initiative-driven and, thanks to the hero, have high defense.
A third-level guild is more than enough to start with. All you need is slow, haste, dispel, shield, stone skin, and blind. It might not be the best set of spells, but it works for me. And then you can capture a secondary or even an enemy castle, which is easier to do with wyverns, and build a stronger guild there.