What is Training and how can you counter it?

Yes, but they limited training for humans to 20 creatures, so you'll have to transfer high-level creatures, and there won't be a swarm of crossbowmen. Hooray, comrades! Hooray, Nival! Hooray, patch 2.1!:D
And they nerfed necromancers, so Dungeon can handle human retraining with its strong units, Sylvan can do it with its sworn enemies, and Academy can do it with magic.