To achieve Air Raider trophy in Dragon Quest Builders 2, you must kill any opponent from above - from the air.
In the early stages of the game you won't be able to win this trophy because you need an item to fly in the air. This item is called the Windbreaker, which you can get from one of the NPCs after completing several main tasks on the island of Furrowfield.
After receiving Windbreaker, all you have to do is press the X button (on the PS4 console) while in the air. You don't have to keep it. Then, when you already use your Windbreaker in the air, you need to find a weak enemy. Once we have located your target, you will need to use the attack button (the triangle on the PS4 console) and wait for your character to fall to the ground. When you manage to kill one of your opponents in such a way, the trophy will appear on your profile.