I did the same thing, but on Timid. I think you took a higher difficulty. There was absolutely NO ENEMY LEFT. But it's quite easy, the first enemies will come to archer range by themselves. You can shoot them from a distance, but they will come towards you and after a few times they will not be bothered with the border. So for your archer's sake, kill them off with either your berserks or FG's. I suggest you kill the Trow with your FG's, trying to keep casualties to a minimum.
From the moment the life-tired villains are dead, your routine starts. To attract the enemy, you can best use the Fetch. Stand out of their shooting range, but not too much, and return once they approach. Stand your FG next to the entrance of your side of the border. My favourite archer position was shift-1 (forgot the name, sorry, short line?) because the never hit each other. Make sure the don't hit your FG's too much, especially on legendary. Once they make a couple of kills the aim will improve, so Trow-killing will be faster. Make sure to get your FG behind the border in time, but not too early, because you want to get all the damage possible. Dorfs will have not much effect on the Trow, so use them only for Myrms, by control-clicking on the ground. If they have to wait for the myrms to get close, they will be stamped to pieces. Also, use satchels only for myrms, and let your archers focus on Trow.
When all is killed, there's an enemy base to the direct left, that shouldn't be too hard. There are still some Trow marching, but surprisingly they won't attack. Attack them with 1 FG or a pack of archers, it takes a while to kill them because they keep moving but it's the easiest thing. There are a few Trow temples with Trow, myrms and fetch in it, but they won't attack either, except when you get too close. Kill them with your archers.
Now there are only a few fetch left, east of the map. You can best attract them with an FG, and kill them with your archers. Also, there are still running packs of fetch/myrms. I first attracted the myrms and killed them with my FG, then shot the fetch with my archers.
Now everyting is clean, except for the other side of the bridge. Use standard strategy to kill the rest and congrats.
Two of my dorfs died tragically because their cocktails bounced off my FG, which caused them to blow up in the middle. I guess Leg could be finished without any casualties.
^ Copypaste this because they are not working here (??)