First of all, you should send your troops in a different place of map, so they don't block each other. You can click on mini map, so it's faster. Move your horses last, since they are faster. Go up, to avoid blocking, then right, then attack enemy's xbows or wait for Barbs to support them. Here's picture:
You need to wait, when two Barb troops (B1 and B2) are in position (together), then move at enemy attacking storehouse (with Xbows as support of course).
Send that one Sword troop (S) and smallest Barb troop (B3) to bottom, Swords can take down Xbows and Barbs can take down some melee at bottom, but remember, to stand in one, large line (3x3 position is not good for Barbs, make it 5 units in first row). Also, make B1 and B2 have 6 units in first row (you can do it while they are moving, so you don't waste time).
It's pretty hard mission, so don't give up and remember to keep your units together, so they are more powerful.
I hope it will help

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