Flying units in Heroes 2.

I think the standard build order for a Necromancer should be as follows: on the first week, 1-4 units with a well, and on the second week, 5 (or possibly 6). Alternatively, on the first week, liches, and on the second week, dragons, with vampires filling in as needed.
In other words, a balanced build order is possible on a reasonably large, average-sized map.
Therefore, on day 121, we have either:
8 vampires (or possibly lords) and infantry;
6 liches and infantry with a magic guild.
On day 127, we have either:
8 vampires (or possibly lords) + 2 liches + infantry, including a significant number of skeletons;
6 liches + 1 dragon (or 3 vampires) + infantry.
On day 131, we have either:
13 vampires + 6 liches + 4 (1) dragons;
13 vampire lords + 6 liches (not guaranteed) + 0, 1, or 4 dragons;
10 liches + 4 dragons + 0 or 8 vampires.
We know there is a dragon lair on the map.
The question is: when and with which units should we go there to minimize losses?