It doesn't necessarily have to be a Strong enemy, or those with a name, to count for this; simply activate your Full-Force Burst ability for your character and then go about hitting anyone around you. As you need to do this for another trophy later on, it doesn't hurt to do this as often as possible; the strength and speed of your character is enhanced in Full-Force Burst, further amplified if you have skills that offer an increase in damage or duration to the skill that caused the Burst. However, you should obtain this through the tutorial mission as well, during the fight with Big Mom or around that time.