Final Fantasy Tactics Walkthrough

You have to prioritize the two Summoners here to find success. They have a long range, and their abilities have a wide area of effect. If Mustadio decides to simply charge them, then the battle's going to become more difficult right away. However, if he uses and succeeds with Arm Shot, they won't be able to do anything but move, giving you the opportunity to kill them both.

All in all, you'll need to kill both of them first because otherwise they're going to be launching powerful summons at you and healing their party members, putting you in a hole. One thing to remember when traveling down the two side valleys is to have your characters end their turns facing inwards towards the Archers because otherwise they're sitting ducks to arrows. Summoners won't use regular attacks so it's useless to face them between turns. After the Summoners are dead, heal everyone up and kill the Archers, then the Knights. You can reverse the order of those last two, but as long as the Summoners are dead and you have a method of healing, the battle will be fine.

The layout of this map also contributes to the battle dragging out, particularly given that the Summoners are separated. It's still best to eliminate them as quickly as possible, from range as described above. Leaving the Knights for last will give you the potential to farm the rest of the enemies for treasure and abilities, as they don't have ranged attacks.

Behind the scenes, another twist and potential betrayal; if you enable spoilers you can learn exactly from what angle.

When back at the map, don't retreat to buy supplies unless absolutely necessary. You'll have the chance soon if you push forward, and the less you fall back, the more you reduce the number of annoying random encounters you might face.