Steven Universe: Unleash the Light

This really comes down to having a character you're comfortable with, and going against enemies with attacks you're comfortable blocking.  For me, that was Garnet, but I also think you could have an easy time with Lapis, as her wave attack will allow you as much time as you want to hit the perfect marker, whereas with Garnet, you only have one shot per round.  That having been said, I recommend only using Lapis' attack on a single enemy at a time, as there will be a perfect marker for each enemy in the wave's path, and you'd therefore have to hit both in a single go.

If you're having trouble, go to Alien Moon 1-3.  Go left 3 screens, down 1 screen, then left 1 screen. (Alien Moon 1-4, up 2 screens and right 1 screen also works).  Kill the light blue boomerang enemy, then keep skipping your attack phase and just block the attacks the other three enemies level against you.  These star-looking enemies only have one type of attack to block, which will make the timing much easier on you.  I can confirm you can take the time to heal your characters with Steven's ability while still in the combo (it has no star meter to worry about).