Annihilation Kill

Kill 10 enemies at the same time.
Getting 10 kills at once will also unlock the trophy for getting 5 kills at once, but, frankly, there really isn't a good place to do this in the story mode; 5 kills can be achieved in one of the handful of trials that you use to practice a few of the skills you unlock from altars in the game (specifically the trials for Earthquake or any of the "massive" versions of spells: Ignition, Freeze, Heavy Strike), but it can be tricky because you usually won't get more than 6 enemies onscreen at one time; during the final level, Dragon Temple, there will be a couple of instances where you'll have the necessary amount of enemies onscreen at the same time but it's very tricky to get them to group together in order to kill 10 at once.

The better option is to do this is in an Outbreak match: absorb enough Chi to unlock the "massive" category of spells: Massive Freeze, Massive Ignition, Massive Heavy Strike; make sure you don't take out any enemy outposts so that there is something to keep your own troops busy/kill them off (to keep them from getting kills you're after), then do one of two things: either let the enemy overrun your forces - simply stay by your base and wait for the enemy forces to come to you -; or run on ahead to the enemy base in order to intercept a group and get first crack at any respawning groups; once you're up against a substantial enough force, melee attack enough to fill your magic gauge and unleash a massive attack - the most effective thing to do is to use either a regular fire/ice spell and follow it up instantly with a massive heavy strike (note that you'll need the Rage Saving ability to pull off 2 consecutive magic attacks).

Outbreak also has the added virtue of immediately informing you of how many enemies you hit with any of your "massive" attacks, thereby giving you a sense of how to develop a strategy for getting this done if you're having trouble.