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No need to make it stronger! It's already powerful! Just look at the ability to summon Pit Lords from the dead units using Archdevils.
My subjective opinion on the strength of the factions (I repeat, this is just my opinion, and you may disagree):
1) Necropolis
2) Sylvan
3) Dungeon
4) Academy
5) Haven
6) Inferno
P.S. It all depends on the specific situation, but this is under equal conditions.