Sentries are a new type of enemy in Lucid Awakening 2. They serve one purpose in battle, and that is to protect the other enemies. Sentries can greatly weaken the strength of your attacks, put debuffs on your party, constantly drain your HP, and all other kinds of nasty things that can make winning the battle almost impossible. Obviously, the only real way to win a battle with a Sentry involved is to do everything you can to destroy it before even attempting to defeat any other enemies.
Sentries also protect themselves, but each Sentry has a weak point. Most will require a specific type of ability to destroy it, fortunately you will be able to see what Sentries weak points are outside of battle so that you can properly equip your party before the fight.
For example, a Sentry's weak point may be its back. The way to take it out would be to have Zeke change to the Acrobat class and use his Leap attack. Once the Sentry is destroyed you can continue the battle as you normally would.
Battles in Lucid Awakening 2 can sometimes be like puzzles. A lot of times you really have to think about the battle instead of the "Attack-Attack-Attack-Heal-Repeat" cycle you see in many RPGs. Sometimes there can even be multiple kinds of Sentries in one battle. You may have a Sentry that guards against back attacks, and a Sentry who is weak against back attacks; the only way to solve this battle would be to take out the first Sentry with its weakness first so that you can attack the second Sentry in the back.