@DavidN
Trolls also regenerate hitpoints at the start of every round (something not immediately obvious). If your troll got even 1 point of health left at the end of the round, next round it starts at 40 again. It can help avoid some of the attrition that the enemy archers do to trolls during long battles (since the AI will go for your most powerful ranged unit at all times).
One technique against Hydras is to try to take them out of the game by casting Blind/Paralyze on them (Paralyze is especially good since they can’t retaliate either). The AI uses that technique a lot. Then you can finish of the other creatures, and then focus on the hydras. Doesn’t help when fighting groups of hydras in the wild, though. 😛
Regarding morale: You get -1 for 3 classes, -2 for 4 and -3 for 5. Since a Knight hero has +1 morale, and since you can usually find some artifacts that boost morale as well, I figure that 3 classes is something you can handle without morale penalties for the most part. 4-5 is probably too much already.
Phoenixes – well, they are very strong, the 2nd strongest in the game (after dragons), and against pack creatures even a small group is probably enough. However, when fighting enemy heroes, I’ve learned that attack/defense skills matter more than one realizes at first.
Consider Paladins, for instance. They are not as strong as a Phoenix, but their production is double, and they strike twice. Take a Knight hero with high attack/defense, against a Sorceress hero with low attack/defense, and a double-sized stack of Paladins will easily make minced meat out of Phoenix.
