If a battle is unsuccessful for an attacker, does that mean the defenders killed 100% of the attacking units?

No, it means you were not able to defeat the defenses.

You have to kill 100% of the defenders to win.

Damage = Damage unit does in a round.
Shield = Replenishes in between rounds.
Health = Doesn't replenish in between a round.

If health reaches 0, unit is killed.