The part you're talking about I'm assuming is the battle where Weigref shows up and kills a knight. It has archers up high and mages on the ground.

Really you probably just need to level up your characters, but otherwise just remember to check your threat range from the Mages and stay just outside of it. I recommend bringing someone (like Ramza) in a class that can get 6 move or so. Then just charge the Mages. I preferred using Ramza as a monk here because it offered me a one hit kill.

I think I know the battle, it's in Chapter 1?

It was that battle where I started perfecting my tactics for combat:

1. Target all healers first: white mage, chemist and yellow chocobo.
2. Target the status inflicters: time mage, black mage and mystic.
3. Target the range fighters: archers, monks and ninjas.
4. You should be left with the slow but strong enemies, so withdraw any squires and mages and enjoy the slaughter

Of course this it just my tactics, and I sometimes alter them such as when I just have to kill one person on the enemy side or in random battles.