How to counter Assassins?
In short: box formation!
The best counter-play to Assassins is a type of box formation, with your units in a tight formation in a corner of the board. Some examples of this formation are shown below in the Mages, Dragon Knights, and Hunters sections. Since Assassins jump almost immediately, opposing units will not move much before the Assassins land and start attacking. This means that if your initial positioning surrounds your important units, the Assassins will have a hard time getting to them.