For this battle, send Erikia, Seth, Colm, and maybe a healer to meet up with Ephraim. Everyone else send up to the door, make sure that Artur is one of them, don't open it yet though. Have Artur kill the Shaman on the other side of the wall.
Defeat the units that come at Ephraim from the way he came, and send Seth and Colm up the path to the Armor guarding the door with the chests. If you want to level Colm, be careful, as the archers will pick him off quickly, as such (depending on if the archers have long bows or not) keep Colm two steps behind Seth so he doesn't get hit. Seth can easily take care of all the units along the path, and the armor at the door as well. Once all the units are defeated, send Colm into the room to unlock the door and chests. Now, destroy the cracked wall and have the units with Erikia and Ephraim meet up with Seth and Colm.
Now, open the door you left your other units by, and send in a character with an axe, I wouldn't suggest Ross unless he's upgraded, kill the armors inside and head to meet up with the rest of the group, send Colm to the room with the chest since none of the armors dropped a key and get the chest. Now you're free to attack the boss and end the level.
This boss isn't that hard, and I've never seen him use a ranged attack so go ahead and range him to death if you want some exp for any particular character. Have Erikia sieze the castle and move on.