The Assault at the Rite of Rebirth will give you one of your first big challenges in Fire Emblem Three Houses, introducing you to the Death Knight and a large group of enemies. You’re placed under the gun in terms of how many moves you can use, and you need to choose wisely to get past the Death Knight.
Avoid Death Knight and stick to your target
Actually, the Death Knight isn’t supposed to be fought at this point in the game. He can easily one shot you and take out one of your characters that cannot die to complete the mission. Instead, on the Assault at the Rite of Rebirth, your goal isn’t to beat Death Knight here. Instead, you should go around the outside of this map to take out the enemies that stand in your way of taking out the mage at the north side of this room.
What you’ll want to do is pick a side, and don’t get to near to the Death Knight as to make him come after you. The standard mages and warriors in this room should be easily picked up if you take your time. If you get into a situation where you’re going to lose a unit, don’t hesitate to use the Divine Pulse to backtrack a couple of turns to make right any mistakes you made.
Once you make it through one of the sides of the room you’ll head towards the center altar and take out the mage to complete the level. Death Knight will ultimately teleport out of the room once a cinematic plays.