Warlock – “Expansion” [4th campaign, 2nd mission]

So, I took a risk and used Summoning for the last spell, and logistics helped me intercept ALL messengers in the second mission (which made the walkthrough much easier). I'm not sure if I did the right thing (if not, I'll probably replay it), but what can you do if there was initially a defense, and Chaos only "appeared" on levels 14-15?