Ok Magus, my suggestion is as follows.
At the start, send all your swordsmen to the left. Axes to the right. Everyone else can gang up on the swordsmaster and the feral one upwards, unless they have bad resistances. The sage in the room with the archers has a couple of neat staves to steal.Thieves are a priority target, they shouldn't be too hard to kill if you hurry. Try not to let them get to the doors. Theres like, 3 more rooms that suddenly appear at the top if either of the two side doors are opened. I suggest taking everyone out before any doors are opened. When you do get to opening the doors, rush every strong person you have into the room and kill them all fast. I think one of the guys in the left room has a mend staff to steal. The only guy in the right room is a sage with meteor. Leave him alone for a bit, he won't move until you get in range. The archer and the priest in the top-middle room don't move at all. The priest can be stolen from easily. The boss is just as easy to kill, anyone can pretty much do it. Just have Mist on standby ready to heal them, she and Ike need all the experience they can get to survive the next round. After this part of the mission is cleared, Ike runs into the next room to face BK. Mist will follow right behind. BK is easy, as long as Ike can survive a round or two with him ( I'm not sure if you even have to fight him, I don't think he moves and he will be killed by Nasir after a bit anyway ). Some thugs will come for Mist after a couple of turns, Ike can kill them easily. I killed the priests with Mist. Just make sure you don't get Ike killed. Easy mission.