How should I develop my units to defeat a Knight?

what's the point of separating them? It's better to place throwers with a higher-level unit there, like a bear or a priest; then, their healing spells will be more effective on weaker creatures. It's very effective to place bears next to dragons; their defense will increase significantly.

Added 2 minutes later
In short, the stronger the "neighbors" standing next to your thrower, the greater the bonus to defense and attack (if there are two healing spells), ranging from 0 to 10.