Is it a bug?

[Fatt_Shade]

I`m not sure i understand you`re question here

I want to ask any a question. All undead must be weak but undead are powerful for my army.

Do you mean undead are strong in you`re army ? If so, then yes, holy anger lowers only defense of enemy undead/demon units.
Ghost 21 att/17def (in you`re army), and 25att/1def (enemy ghost) is this what you saying ? It`s OK ,that is how holy anger skill works.
You can lower other enemy units stats :
1)dragons with shield of recross , dragon tooth necklace , eye of dragon will lower enemy dragon attack.
2) ray of light, silver rapier will lower enemy undead attack,
3) torch will lower enemy plants defense . . .
So there other things in game that lower enemy stats, to help you cope with them, if you get problems with certain battles.