Orcs vs. Humans

Under these conditions, it's hard to give specific advice. Try focusing on mentoring, and for your secondary heroes, take the skills "Protect Us All" and "Goblin Support." By the final stage, you'll have a huge number of goblins; use them to set traps, and use the "Wrath of the Horde" skill with your main hero. This should allow you to effectively defeat the knights with that army.