This was a great chapter. Natsu activating Dragon Force, making combo with Gray to kill Mard Geer, and we get to see next week continuation from Erza and Kyouka fight. I enjoyed it.
Also, I wanna add that it didn't seem like Natsu activated Dragon Force at will. Think about it, he went Dragon Force mode only after he thought Gray died, and before with Juvia vs Meredy we saw how emotions can increase their magic power. It is not that illogical at all. We know that Dragon Slayers need substances of high magical properties to enter Dragon Force mode. I am speaking strictly about first generation here, because second and third have lacrima in their body and we simply do not know enough about pure Dragon Slayer. So, say that in order to activate Dragon Force, they need a high ammount of Magic Power which they do not usually posses. After Natsu got angry at thought of Gray being dead, his power skyrocketed enough for him to achieve Dragon Force temporarily. Again, this do not hold water. This is just one of many possible theories.