Galthran is generally the starting hero due to 3 stacks of skeletons and skeleton specialty, allowing for good tempo in the early game. He’s also the best for doing fights when skeletons are your main army.
Isra and vidomina are good for skeleton farming due to their necromancy specialty.
Thant is good because he has animate and also specializes in it. In combination with scholar this can help not have to depend on rolling animate in the mage guild.
Any spellcasting hero can also be good in case the map doesn’t give you enough casting stats.
First two turns are almost always skeleton transformer>mg1
After that it depends on the map
It is common to go for a necro amplifier turn 3, but if there aren’t too many opportunities to necro up skeletons on the map, then players can skip it in favor of getting mg3 sooner.
After the necro amp (or after mg1 if necro amp wasn’t built) it’s most common to build mg2 and 3 to get animate, unless you have thant + scholar or are going for vampire lords.
If the map gives you enough vampire dwellings (usually 3 or more, but at least 2), or if you find a vampire/vampire lords box, after necro amplifier you can go for estate and upgraded estate as enough vampire lords can farm the map without the need for animate.
A less common option is to go for black knights or liches if the map gives you a few of their dwellings.
Animate dead - usually a must have for necro. Unlike resurrect, one doesn’t even need a magic school to keep the raised skeletons after combat.
Teleport - can be really good as skeletons are slow and can be kited easily. More effective with water magic, but generally works well without it too.
Haste - can be a big help for the same reason as teleport. Most effective with advanced or expert air magic.
Other useful spells - this is pretty much the same as for every other faction so I’ll just list them without a description: mass slow, mass shield/stoneskin, good battle magic like implosion or chain lightning), control spells like town portal and dimension door/fly.