Here comes a somewhat twisted idea, never tried it though.
First: An infantry unit charges a cavalry/monster unit and they make sure that there is a wood or the like nearby in their direct rear and within a few cm.
Second: Another unit, whichever, charges another enemy and they make sure that the point where the first unit may withdraw into the wood is within 30cm.
Third: the dragon joins the first unit and they start the combat.
Combat: When the first unit withdraws/falls back into the wood leaving the enemy cav/monster outside, the dragon leaves and joins the other unit within 30cm. Then the second unit start their combat.
Correct?