Galthran is a hero. His specialization is skeletons. He can bring up to 90 skeletons with him. Necromancy raises 30% of skeletons. But this percentage can be increased – a necromancy enhancer in the city (+10%), artifacts from which you can assemble the Cloak of the King of the Dead (+30% to necromancy; with expert necromancy, killed units are raised as liches). There are two other heroes specializing in necromancy – Vidomina and Isra.
On the very first day, it is desirable to upgrade necromancy to expert level using chests and stones. Until vampires and liches appear, it is advisable not to attack ranged units. But low-level infantry units should be eliminated. This is to increase the number of skeletons. Black knights are also effective. Dragons are weak. Many don't even build them. In my opinion, it's better to build them.
In general, the main thing is the number of skeletons.