Alright, more detailed help. The faq is here , but if you want specialized detail, here it is.
(When I mention Vaida, it is only if you didn't kill her in an earlier chapter)
First, have most of your party move north(north is up, south is down, west is left, east is right), but leave eliwood and two other guys in the bottom corner where you start. They need to talk to Vaida when she comes, to have her join you. As the people move north, dispatch the enemies, and watch for the people spawning near the fort at the top of the map. When everyone there is gone, move west, watching for any big enemy groups. If your weapons break, try to have Merlinus as a caravan, so he can follow you. Keep magic users behind the frontlines, for support. Just keep moving forward as you remove the enemies, have Eliwood talk to Vaida when she comes, and stop at the armory and shop to re-stock before you take on Lloyd (or Linus, depending on which one you fought before.) When you get to them, use magic users who won't die to weaken him, then have powerful close-range units to finish him (if you want to use the Ballista, go ahead, but its not very useful; it misses most of the time.
Another method is to have everybody but about 5 move west to the second "Island" and have your best block the bridges, with magic users for backup. Send a flying unit to the city in the west corner to get a Warp staff before the brigands can destroy it. That flyer can buy items from the armory and shop and send them to Merlinus, so you can use them early. For those on the east "Island," send the powerful ones to the the bridge, and have Eliwood and two others to the corner, to wait for Vaida. As the enemies attacking the "islands" diminish, move most of your forces north, watching for anything dangerous. I like to use a paladin (Kent) to clear out the area with the two ballistas (have weak units, healers and such, avoid the ranges, the ballistas are very powerful,) and then clear out the open area in front of the boss (move him onto a fort.) Then have the rest clear out the middle of the field, using Psychic on the paladin as necessary. When Vaida comes, have Eliwood talk to her, and use her and the ones you left there to take out the other two wyverns (three more will come next turn, so don't leave yet.) The rest should be pretty simple, take out the other remaining enemies, atop at the stores if you need, and move on to the boss. The same rules apply for the boss as in the above strategy.