You've got the nice visuals down, mostly. There's parts of the concept that are nice too.
That said, the practice mode/group mode distinction is totally pointless. There'd be no difference in challenge if the player could just play one level at a time to unlock more levels, and it'd be more considerate of your players that way. Some players do have lives to get back to.
Further, the requirement to immediately start aiming once a retry begins regardless of where the mouse was before is infuriating, and the design of most of your levels is so fiddly that half the time the aim indicator is wrong anyway. This is only made worse by the enemies so often just making the range that will succeeded unnecessarily narrow. If you really had to use the "one second" limit, it'd have worked better to give the player only 1 second to see the indicator, but unlimited time to just look at the level and figure out what they're going to do.
Aside from that, the crystals look like background rather than anything important.