The stat increase that can be applied by Def Opening is considered a Bonus.
Any 【Bonus】 on a unit, including Bonuses, lasts until the start of that unit's next phase. If Bonuses are acquired during the enemy phase, they will disappear when the next player phase begins.
Unit selection takes place after all Bonuses on allies are removed, and before any new Bonuses or Penalties are applied. All other existing Bonuses and Penalties count towards their respective stats during unit selection.
Def Opening does not compare Def stats, so 【Phantom】 effects will not work.
If multiple units are tied for the highest Def, each of those units will receive the effect.