I don't think army points even matter. Hard fortresses have been pretty wonky, in the aspect that I have 12k assassination army points, mind you I don't equip these, and my chances on the last 20 fights on hard have been 13 wins - 7 loses.
I have invested quite a bit on seraphim's though, with trap bonus +8.25 %. So that might be a huge factor on my wins. But still, these fortresses defense points should crush my army.
So would be nice to have an explanation what the potato is going on.