Easy, just have a storm pop up. If he flies, he's going to be contending with hurricane force winds blowing him too far away to be in range of his target, and taking damage every round from being battered by wind and lightning. In general, having a storm raging around the battle lets you do lots of neat little things that'll make the encounter more than a stand up fight (sliding combatants around randomly, damage all over the place, poor visibility, etc)