Why he wouldn't have chance? I just think that, let's say... 4 regular troups at the time is enough. Two battalions of Erebor warriors, two of axe throwers, two of mirkwood warriors and two of archers. Or... they could be summoned in spell that you said you will implement, with gimli and legolas.
So, it wouldn't be making defender overpowered, remember that's commandpoints limit as well. This units could just take more CP.
But... it's only suggesion for you to consider.