It's a bug. 9 units is the most you're ever supposed to have in a single army, but sometimes there's an issue with taking garrisoned armies out of cities that results in the city's permanent garrison being carried with the army.

As far as dealing with it goes, I'd suggest building up a bunch of cheap units and wearing them down, or shell out a few hundred gold for a couple of Blizzard scrolls at a Conclave with the Scroll Scribe improvement, or bring in some high level champions with good weapons or, better yet, magic to rain death upon the enemy.