AoS 2k slann and kroak

Are you talking about Dracothion's tail?

Instead of setting up a unit from this battalion on the battlefield, you can place it to one side and say that it is set up waiting to appear as a reserve unit

In your hero phase, you can set up one or more of the reserve units on the battlefield, more than 9" from any enemy units and wholly within 18" of the Slann. However, the same unit cannot appear more than once in the same turn.

 

I just posted a possible start for such a list over in another thread:
http://www.lustria-online.com/threads/is-mortal-wound-spam-possible-now.23279/#post-281372