Fairly straightforward- fly your drake through a bunch of orcs and kill them with fire. Your bonus objectives are to kill their captain in the carnage, and do it all under the time limit.
The captain's no problem- just hold down the button. As for the rest, the key is not to get too fancy. Fly straight to the nearest orc clump, hover in place, and hose them all down with searing death. Do check to make sure they're dead before winging on. Do keep an eye of your might and chuck a fireball at distant groups when you can. When 35 orcs have been comfortably charred, the mission completes.