How do you banish monsters in gemcraft chapter 0 gem of eternity

Monsters are "banished" when they reach your wizard tower at the end of the path. The monster is returned to the start of its' path, but at a cost to your mana. If you do not have enough mana to do this you will be defeated!

Each time a monster is banished, it requires an additional 50% of the original banishing cost, to banish again. This can add up quickly, especially with fast monsters.

You can add skill points into the "Armor" skill to lower the initial banishing cost of the monsters.