Finally got around to finishing this fight. Like Chocobo said, destroying the north bridge is a great help; engineer bomb, crossbow bomb, or a ice+fire spell combo will do it. If you destroy one square on the "close" side and 2 on the far side it gives you a bit of liberty to range harass the calvary; since they aren't smart enough to go the long way while having a strider in the start area. The temporary helper shows up around turn 4-5, but he'll only help for this battle (He's a male swordsman who also knows weak fire magic).
Since the AI's aggression is very low, dividing and conquering will have the best results. Walking around the top of the first house to your left or sending flyers (I gave my new cavalry guy the helicopter pack) north of the west bridge allows you to dispatch the flying force. Shadow resistant units make the best lead ins: park them 1 square out of their melee range which'll force them to use their breath ineffectually, then use piercing/light attacks to slay them quickly.
The Mantis knights are probably the largest threat on the board, their charge hits hard and they have lots of hp. Present a flat front of melee to prevent them from double teaming units and use Sabrina's traps liberally. The crossbowmen and healers behind them will clean up fast. Now you should be able to get the chests in the two houses without issue. During this part of the map I found the Shadowport skill of the shadowling useful for traversing through the house walls.
Group up before passing the 2nd treasure house. When you approach the boss he'll likely use a charge attack, thankfully outrunning his mentalist entourage. The back airforce will likewise too long to reach you before you kill the boss.