The Supply Stash was gone from the game for a short time; while it continues to be available, the limited-time flyer it provides is invaluable for this quest. Killing flying enemies is exponentially more difficult for players on the ground.
However, there is an effective, if not easy, solution. It starts with the obvious tactic of using the Nature's Vengeance spell. But the Mystic would have to operate at half capacity, the pet unable to take to the air unless its master is airborne, too. Seekers are of course even worse off. So the trick is to use the Direct Damage spells as a lure. Because all enemies make a beeline (excuse the pun) for their target, they will gradually lose altitude as they follow the luring player. The pet will automatically, usually, attack the flyer as soon as it is low enough; Seekers will have to judge the distance while running away, and double back to intercept their enemy.