More discoveries this week, which are probably already known.

But just in case!

reveals the order for killing enemies for the large crystal. The first one becomes the blue crystal, the second green, the third red, and so on. Might be slightly useful if it happens to appear on a level where no one knows the order yet. (I assume that transforming the enemies in Ninjutsu mode works the same way, although it would be harder to tell where the skull-marked enemy fits in the sequence.)
If you freeze an enemy right before a special item kills it, you'll get a normal blue crystal instead of a diamond. I've seen it happen with

and

so far.
It's possible to complete CRISTAL without ending the level. Just today, I grabbed the letter
I right as the last enemy (a big apricot) fell offscreen, gained a life, and got a head-start on the next CRISTAL by collecting another
I prior to knocking off the two little ones.

I wasn't expecting it, so I wasn't paying close attention to the timing, but I'd guess that taking the last letter
after big apricot falls offscreen but
before its little ones appear is probably the key.