The script is here: https://greasyfork.org/en/scripts/35031-wanikani-leech-trainer.
The script will put together a lesson of 10 leeches. It will exclude any leech that is too recent (less than 24 hours since the last review), or too old (less than half the time between reviews remaining), or already trained. Any trained leech that is subsequently reviewed incorrectly goes back in the pool.
This is more of less what it looks like (this animation is slightly out of date):
Every leech is repeated 3 times in the lesson. To clear a leech you must get it right each time. Furthermore, the lesson will include up to 3 items related to the leech. You must get those right too in order to clear the leech.
Leech training data is stored on the server-side. So you can switch browsers or computers without losing track of your progress.
As always bug reports and suggestions for improvements welcome. Enjoy!
PS This script steals heavily from the awesome [Userscript] Self-Study Quiz If you haven’t installed that script you should go ahead and do it now.