How to balance a conflict between animal people when one of them has the advantage of flight?
Setup:
avians are a humanoid species with bird characteristics, they all posses the ability to fly. Just as other species they can learn to use bows, swords and magic. Their main fighting tactic is using long spears to charge down attack and fly back up, they are aware using bows from high above would be safer but they are reluctant to change their old ways but will change if things get too bad. Their aesthetic and culture is vaguely based on Qin dynasty China.
Leonins are a humanoid species with lion characteristics, they posses enhanced strength. They are also capable of use of all weapons and magic. 46
Their main fighting tactics are their world-wide famous unbreakable shield walls. Their aesthetic and culture is vaguely based on the Roman Empire.
Rules:
* They both have access to magic but they are both bound by an international pact forbidding them from using magic for large scale destruction on the battlefield, but they are allowed to use magic for healing and strengthening their troops (main examples being making troops bigger or smaller, magically protected, stronger, faster and braver).
* The avians have most of their important cities high in the mountain making them difficult to attack but they have while the leonins live in the plains south of those mountains but have more territory and people.
* The conflict started because the leonins took in a price that failed to set up a coup. This is the excuse for the war but the avians are interested in expanding their territory, weakening their neighbor, in glory and in their prince. The leonins originally wanted to destabilize the avians but now want to prove their strength and keep their pride, they also want a bit of glory.
Question:
I want this conflict to generate a big number of refugees from both sides and a large amount of animosity. In my view the conflict seems to favor the avians heavily making it an easy victory due to their abilities to fly over the enemy's armies and walls and to attack from a distance with long spears or bows. How can I balance this and draw the conflict out into a long and bloody war, preferably displacing many civilians? 261