Explain me this
The necromancer stats are different in defense
You need to lookup the stats of the unit by selecting them in the guard tent building (where you assign the waves).
The stats there are the ones used in defense and are indeed different that the one from your barracks (used for offense).
What you see on Troops Academy are the stats when you use Units to attack.
You gotta go to Castle Guard and check the Units when you are adding them to your waves to see the stats they have when defending.
Troops Academy / Castle Guard
Some troops and towers have weakness to differents spells.