Flying units in Heroes 2.

Ideally, the hero should be decent, preferably a Knight with stats like 6-7-2-2. A simple Necromancer, like one with stats 1-0-3-3 without Animate, and with 90 skeletons and 4 dragons, will suffer heavy losses. You'll have to take down the Black Dragon in 3 hits.

Yes, after some consideration, the option you suggested also seems like the best one.
For example, everything can be implemented like this.
Start with a Necromancer. Buy a Warlock hero, and send him with one Gargoyle for scouting (which is what he's good at). The Necromancer quickly levels up Necromancy to Expert. Then, start gathering skeletons. As soon as you build the first guild (which is needed for Liches, and they are needed for Dragons), buy a Knight. Buy him a spellbook, put him on a Gargoyle, and send him off to level up. The main Necromancer and the Knight meet at Dragonspire. Give the Knight 3 Centaurs, 1 Gargoyle (if not, a fourth Centaur), and 100+ skeletons. With a good outcome (200 skeletons and low morale for the Dragons), the Knight will only take 3 hits. After that, the Knight gives the remaining skeletons to the main Necromancer, and he himself, with the Red Dragons, continues on. Later, Bone Dragons will join the main Necromancer's army, and Vampire Lords will join the Knight's army. Thanks to the Expert-level Leadership at that point, you can recruit troops in other castles (but with a speed of 5+, for example, Druids, Grand Elves, and of course, Gargoyles: for the first turn and to remove retaliation). It's best to carry out the "capture of Dragonspire" operation on day 137; then, on day 141, there will already be 3 Dragons.