Tactics for the Swamp (Fortress).

I'm talking about the "Forgetfulness" spell, which for some reason isn't mentioned in this thread.
And in general, it's an extremely useful spell. With it, having only one dragonfly in your squad, you can defeat any ranged unit (except liches and titans) even with Goblins, with almost no losses.

Quiet horror. To spend 2 slots, 6 skill points, 3 days of construction, money and resources on something that, against Serpent Riders (and sometimes even on level 11), or Tactics (which is also not uncommon), can be defeated anyway, and after 2 weeks, having Hydras and other dragons, you have to fight with Goblins.