Steven Universe: Unleash the Light

Lapis starts with her Tidal Wave attack already unlocked.  It damages enemies in a straight row, so you will want to find three of the same enemy in a row and use her attack to take them out.  Unless you're really good at hitting the perfect attack bars in her attack, do not try for them while going for this trophy, as you'll only deal extra damage to the enemies whose perfect bars you hit, and for this trophy you'll want to do consistent damage across all enemies.

There's a decent place to unlock this in Alien Moon 1-8.  From the start, go left, then down, then right.  You will reach four small bomb enemies and one large one.  Use Garnet to take down most of the big bomb's health, then either successfully use Lapis' focus ability, or use Steven's Shield Throw to take down some health of the bombs in front of the big guy (depending on your levels, this may not be necessary).  Then put Lapis in the back row and unleash her Tidal Wave attack at the row of three.

Alternately, in Demantoid's World 2-8, if you take the left elevator up to the second floor and go down 2 screens, you will encounter a circle of small bomb enemies.  Place Lapis in the top row and you should be able to hit all three of the top ones.