In Fire Emblem: Three Houses (FETH), you’ll face many opponents as you progress through the game and they will vary depending on the house you pick at the beginning. If you pick the Blue Lions house, you must face Cornelia.
Remember that you’ll not be alone in combat, since you’ll have many soldiers and armored titans, who can make things a little difficult for you. It is best that you take the Titans first and then attack Cornelia. Another tactic that you can follow is that while more of your mobile units fight against the soldiers, the strongest members can dedicate themselves to attack Cornelia.
Keep all members of your team with loaded health, as opponents’ attacks can be devastating to your troops. During the battle a thief will also appear, whom you can defeat and you’ll get the extra reward of a special bow. If you follow these tactics, then it will not be difficult for you to defeat Cornelia.