Remember to save frequently.
Upgrade all your infantry techs in the blacksmith and barracks. Speed and arson are good. Sell all your wood slowly because the price seems to improve quickly in this mission. Buy enough food to use all your gold training huskarls. Attack red, kill the gates and kill the army. Destroy the town centre.
Train up more huskarls, sell your wood. Buy enough food to buy all huskarls with the remaining gold. Purple will attack soon, on one playthrough they actually destroyed my initial base which is very annoying.
Park your dudes in the exit of red, send 5 huskarls to attack the nearest yellow gate. Lure his scorpions and skirms out of their town. Kill the scorps and run through the gate when their skirms open it for you. Make a dash for the upper gate, destroy it and destroy the town centre.
Train more huskarls. Sell extra wood and make more huskarls. Keep 8 behind in case purple tries to attack yellow or red. Send 5 huskarls to attack the gate of blue. Wait for their army to come (especially the onager). Rush through the bottom gate when a crossbow opens it for you and kill their army. Kill their onager before it can kill your dudes. There is a monk worth killing. Once both are killed, kill the second gate and kill the TC.
Train up more Huskarls. Sell wood, buy food. Train more huskarls. Free the gold markets with your huskarl ball.
Train more huskarls, sell wood, buy food. Move on cyan. Cyan has a smaller army but has a castle. Wait for their army to appear at the front gate. Kill it and run past the castle to the second gate on the right. Destroy the gate and kill the Town Centre.
Sell wood, buy food. Train huskarls. You should have good income now. Send your huskarl ball to clear the mines. Lure them away from the towers for minimum casualties.
Sell wood, buy food. Train huskarls. Spam 60, send them to the gate of purple. Start attacking. When Purples army shows up, move inside and get into the second gate when some purple unit lets you in. Kill the town centre plus towers and move south to find the rest of the villagers. This will stop purple producing many units. Then move north and take out the production buildings.
Meanwhile, sell wood, buy food and train huskarls. Next 60 unit ball, send to kill the gates near the castle. After the gates go for the castle.
Purple might keep attacking your old towns. I find that 8 huskarls should be enough to kill these groups of rams. Good luck with the achievement. It's essential that you only train infantry so good luck!