Your favorite hero...

No, the avatar is stronger than the phoenix. It has a lot of the best Necropolis skills, and its stats grow faster. The only downside for both types of units is their vulnerability to the cleaving sword and the special ability of the Death Harbingers.

Anyway, ghosts are easily defeated with a large army. Moreover, you can simply ignore them and attack with the hero's spells. So, what does Vlad do?

I don't know about higher levels, but at the beginning, the avatar is of little use. At level 1, it costs 2 mana, while Arantir costs 10, so I sometimes summoned it, but then its cost increases, and unfortunately, the mana doesn't increase:(. Therefore, the higher the level, the more difficult it is to summon. At level 20, it will cost 40 mana, and the Necromancer will only have 40-50 mana by that time, if he doesn't take the education and mana attraction skills. And the phoenix costs 35 mana, and its stats depend on spell power, which the Necromancer has a lot of.

P.S. Vlad enhances the Raise Dead skill.