Originally Posted by Maladaptive

Anyone know how/when to use the Crystalis items?

In the event exchange shop, if you scroll to the second listing (arrow button on the right side) you can trade 160 (!) of them for a slayer girl for the next event.

If you really want that slayer girl, then get as many crystalis as you can, but keep in mind you can also buy extra crystalis (if you're a few short) in the regular store.