Try playing this mission and winning all the battles without any losses... You can also try lowering the difficulty level.

I'm playing without losses, with 12 Titans, 40 Mages, and 400 Goblins, plus some other units. The demon has 100 red Unicorns, 50 units of the second-to-last level, and 400 Hellhounds, with an attack of 20-30 (not damage, but attack). The difficulty level is the lowest. These red horses kill any of my units with a single hit. After the first round of attacks, I have only 5 Titans left. Their numbers are almost unchanged.
I'm wondering if the demon attacked everyone at the bridge, or if I should replay the mission.