Dorter Trade City is a common stumbling block for many new players, but the battle is quite fair. It's the first wake-up battle in the game -- a test to see if you've properly learned the gameplay mechanics. The first battle that will get you slaughtered if you try to brute force your way through it. Tell me your party and what you're having trouble with and I might be able to help.
The whole battle seemed to be designed to be unbeatable. A whole bunch of long range enemies, combined with putting every one of them in an advantageous spot. Also, mages are beyond broken. I thought that their spells could miss you if you get out of their range before they manage to cast them, but that is definitely not the case. You also only get about 4 people and 2 guests for the battle, which was complete BS, especially since the guest AI is idiotic. I had my main guy (Ramza, a squire), a black mage, another squire and a chemist, along with the two guests (Delita the knight and the other guy whose name I can't remember, who I think was still a squire).
We were massacred, every enemy hit was basically a 2 hit KO, and my chemist couldn't heal everyone. Since the enemy could hit me from about half a screen away, and only Ramza had a decent amount of movement, it was probably the most one sided battle in any game I've played recently. Even when I hit them they had about 80 max HP, while my guys had about 35-50 HP, so they were always going to beat my guys in basically every category.
I've decided to just move on. I have a bunch of games on PS3 I can play, I'm not going to waste time on something designed for frustration over good gameplay. I really like the idea of the gameplay, but FF Tactics was designed by sadists.