Grinder?
 

Any ideas how to get this?

EDIT Got it now
Basically move a ball (or whatever icon you're using) to another ball which you know WON'T allow you to finish the level and then press undo. I repeated this quick step about 20 times then finished the level as normal and up it popped. Took about 30sec on 1st lvl of freeplay with 3 balls

yep did this about 10 times and it unlocked.

Weird, won't unlock for me. Tried undoing the move 10, 20, 30 times and finishing, but no joy.

The 15 level achievement will not unlock for me, I've played challenge mode level 1 to 6 playing 20 levels with no wrong moves and nothing