A battle from 2 different sides. You'll see more like this later on. Where you have position your units to fight from 2 or more sides. The white mages and Archer's make things annoying. Since they're not limited by the giant wall. Although you might get unlucky and find that the Monk has Wave Fist which is also ranged. You can use your strongest...or really tankiest unit to act as a temporary wall to prevent the others from walking out the abandoned building. Or you can utilize archers and mages to annoy the enemy harder. Just know that the enemy is smart and will take advantage of mage spells. But likely you have Summoner so you can abuse that cause summoner's have a far better AoE and they can't hurt allies.
Algus and Delita...well they like to do what's opposite of your favor. Don't count on them to do anything except kill weakened enemies.