I played Orleans on normal, and it did take a couple of tries for me.

Here are the tactics I used. I made one group of spearmen charge out the gate and engage the knights to the east. I then had 1 group of knights (A) come out followed by the 3 groups of crossbow men. Your knights (A) may need to charge the English knights to help out your spearmen.

Now comes the tricky part. You have to get one group of knights (B) out to attack the west balista. After they get out of the way send out the rest of your troops and set them up in a defensive position in front of your gate (with xbows out first). Their job is to delay the enemy, and keep them away from your 2 squads of nights A and B.

After your knights (B) knock out the western balista have them take out the northwest one. Hopefully buy this time you knights that have engaged the English knights are free and can run and destroy the eastern balista. Then attack the north east balista with whatever group of knights (A or B) can get there quicker. After the balista is destroyed you will have won the mission.

I hope this helps.