You'll meet it in chapter 3. You have to kill him in order to get through the gate. He has a guard of two wolves. You need to get rid of them first - they deal more damage than the boss himself. The Giant Ogre after an attack that's heavier than a sword strike uses a fast attack, which doesn't change the turn cooldown time.
Don't waste Rubella's turn for attacks, she's more use to you healing Aurora. The protagonist's attacks are much more effective, and the Ogre can take even 30 HP with one attack. If you can, set up a shield guarding both characters - you won't waste your turn and you may aid Aurora.