Finally you'll get the chance to end the lives of the revolting characters in the Nohrian army. However, don't expect it to be easy. Iago is wielding a Rank S tome called the Excalibur and has several debuffing/status ailment staves. Hans in the other hand has a rank S axe called the Aurgelmir. There are five chests in the map, one on each side and three in Iago's room. Your objective is to seize the throne so even if you wipe out the enemies, you'll have more than enough time to open these chests. The rooms are separated by doors that can be opened by locktouch or destroyed.
Chest contents:
The western side has Faceless and Stoneborns so characters with Blessed weapons should be able to wipe them out easily. The eastern side has mages so characters with Tomebreaker can do well in tanking their attacks and pretty much powerful melee units with high RES can take them out in one blow.
As usual, you have to divide your forces to two with the same setup; at least one main healer and backup healer for each group with a locktouch character each, and more importantly, tanks and damagers. Fight your way through the rooms until your team is able to regroup in the southern, middle room. If you're confident about your individual group's strengths, you can go ahead and move them in; otherwise, you have to wait for both of them are at the southern room's doorsteps so you can open the doors within your turn and launch a pincer attack.
Take note that opening one door will make all the enemies in the southern room to flood in and focus their attacks so it's a good idea to keep your most powerful units at the doorstep and create a bottleneck which will allow your ranged attackers to deal backup damage while giving your healers chance to heal your frontline fighters as well. Your mages will be the bane of these pesky generals but be careful when using mounted units since some of these enemy generals have Beastkiller equipped. Once your second group enters the room, the enemies will divide their attention and get scattered, giving you much needed breathing space. If you need to save, do so when you're in this point of the mission.
Let the filhty Iago use his staves; he'll only get once chance per turn anyway. When you're knocking in his doorstep, destroy the gate using a ranged attack instead of opening it so most of your units can move in and swarm on him. To be sure, it will be a good idea to save at this point. Once the rat's gone, you're free to loot the chests and seize the throne.