Flying units in Heroes 2.

It is generally not advisable to take Liches and Vampires.
If you can find a good stack of Orcs or Archers, the following tactic might work:

Dragons
one archer
skeletons
main archer stack
one archer

Round 1: Cast Stone Skin on the dragon stack and use them to absorb the initial attack, while the skeletons finish off the enemy.

or

1 gargoyle
1 gargoyle
1 gargoyle
skeletons
dragons
Stone Skin on the dragons and...
Attack the red dragons from both sides (from the black and green dragons) and one green dragon (from the red dragon).
If the skeleton horde is good and there are 2 or more dragons, 2 dragons may die in the first round (red and black (or green)).
Then you will have to fight the rest.
I think you can limit the loss to 1 bone dragon.
If the army is approximately 90 skeletons and 4 dragons, it may be possible to win without any losses using the first tactic.