This can be easily unlocked outside of battle during your first visit to Port Maerifa.
As you progress through the storyline in Port Maerifa you will unlock the ability to use a spear outside of battle by pressing
. At this point, some enemies will also spawn inside of the city.
Find one of the enemies and tap
to chuck your spear at them. Repeat this until you get to a combo of 108 or above. The achievement will pop shortly after the combo ends.
I'm sure this can be done inside of battle, I was just surprised to find such an easy and early method.