The Two Ai units in the photo have killed over 100 of my units slowing me down and letting other nations get stronger. they are invincible. I even have a spy in the province and it doesn't give me any intel on the army.
A lone intelligence spy has a low chance of showing the amount of units and the level of buildings. (i dont know what the chances are)
Personally, I would recommend having about 5 or more Intelligence spy's for that. As for the loss of units, I suspect your looking at level 4 or 5 fortresses, each having a decent amount of units. A handful of Artillery would be my biggest recommendation for dealing with the fortresses