Defeat 100 enemies. This includes enemies on encounter tiles, bosses, and AI player characters.
Enemies and characters can be chain-killed in the same match and still count as additional 'defeats'. Human characters fought in multiplayer mode do not count, but any CPU characters or enemies defeated in an online match do.
To grind for this quickly, it is recommended to have a high-attack character like Yuki or Tomomo, play on maps with the Battlefield effect enabled, and simply kill everything in sight until you or a CPU player wins the match. Rinse and repeat until the achievement pops.