After an attacker assigns damage, in order to keep a unit I just put out last turn can I place the damage to the zone and not put any damage on the unit so he stays alive? Or do I have to use him as a defender?

If I can keep him alive it helps give me loyalty points so I can bring out tougher units to deal with the attackers.

The battlefield phase order goes (actions can be played between each of these steps by both players):

- Active player declares where they want to attack
- Active player assigns which units they want to attack with
- Defending player decides which units in that zone they want to defend with
- Damage is assigned
- Damage is applied

If you want to keep your unit, don't declare it as a defender.