What anti-air do you have? Because you'll need something with splash or Dwarves will be annihilated.
The idea of dwarves is that you stomp the enemy base
before they get the chance to build air units by sending wave after wave of heavy melee units
The dwarves have miners which can gather gold/stone at a much faster rate than magic or tech (I actually had to make them gather less after testing!)
This allows them to churn out loads of warriors and heros, which require gold/stone.
In games with 3 AI opponents its possible that 1 may get some air units before you can crush them. In this case just build a few Dragon slayers. As the name implies they are specifically made to destroy dragons (or tech air units).