Final Fantasy Tactics Walkthrough

The battle may seem tough because of the number of enemies, but they're all just regular monsters and are easier to defeat than human enemies.  The best job classes to have here are Archers and Black Mages, because Archers can attack from long range and then run, and Black Mages can deal a lot of elemental damage, also from range. Specifically, the goblin-class monsters are weak to Blizzard magic.

If you've been struggling with battles so far, fight a couple of random battles at Mandalia Plain to level up your existing jobs so you can change to better ones, and make sure that if you bring in Archers, you've equipped them properly with Bowguns. This is even better if you can make your two guest characters into the Archers, because that will change the way they think about going into battle, often keeping them alive longer.

It doesn't matter what order you defeat the enemies in as long as you kill the Bombs quickly; if you don't, they'll self-destruct and cause a lot of damage.  Once again, it really helps to have a Chemist so you can heal if someone takes too much damage.  A White Mage also works well even though they have to charge their spells and Chemists can use items instantly. If your Chemist is sufficiently advanced, a White Mage with Items as a secondary skill and the Throw Items support ability will allow the White Mage to be even more useful and also build up JP to unlock magical jobs further up the job tree.

You may want to consider doing some more random battles if you've been struggling still, as the next battle has another local difficulty peak. Having Move +1 for the next battle is very helpful.