Tactics for Dungeon.

The race is decent, it's fine. However, many of the monster structures require valuable resources, and sometimes there's a shortage of them. Also, a guild of mages level 2 is needed for Dragons, and level 3 for Black Dragons. Mana Vortex is great (I've never seen the AI build it), and the Summoning Portal also helps with extra warriors.
Regarding tactics... I don't play Dungeon much, but with 2.5 ranged units (0.5 - harpies), I usually stick to a defensive tactic in battle. And it's convenient to storm castles with only 2 infantry units, the rest being ranged and flying units.