DO NOT TRY THIS ACHIEVEMENT ON HARD DIFFICULTY WITH THE BELOW STRATEGY.

Teal is an infinite unit spam AI in his mission and Wallace is surrounded by military buildings. On Hard, the AI will nonstop spam Champions, Cavalier, Siege Rams, Light Cav, and Crossbowmen. On Standard difficulty, there will be a massive delay before the AI upgrades from Longswordsmen to 2hs and there are massive delays between unit creation. I tried hard, but I got overwhelmed every time. You'll likely lose most of your units trying to get to Wallace, who is leashed to the general area, so can only slightly be kited. Even if you destroy the military buildings around him, Teal has more further back and by then, you've likely lost most of your army where the trickle in from the back is still too much.

While it is possible to do it on hard, this guide is focused on standard difficulty where you aren't microing a mile a minute.

Do all of the scripted stuff. Hold off against purple's advances. Try to lose as few units as possible as you'll need them later.

After 5 mins, immediately go to the castle in the upper right corner of the map and research warwick. While this is researching, have all your units on the bridge healing up with the 2 monks you were given. Once completed, send the hero treb in to destroy the gate.

Once destroyed, lure out the units in Sterling in small groups in order to properly counter them. Send your hero right away to kill the scorpions whenever they aggro. Once again, you want to lose as few units as possible.

Once Sterling is taken, you'll get a scripted scene where Longshanks will warn Robert what happens to traitors and everything will turn over to your control. This starts an invisible timer before Robert will ignore that warning. It's sometime between 10-15 mins.

Immediately research Bracer at the blacksmith and Cavalier at the stable. While researching, go to the 8 stone and wood villagers you protected at the start and send them and your entire remaining army north of William Wallace's location. You'll want to build a castle here. Warning: If you build it too close, Teal will aggro with his entire army.

While building it, do not attack any of Teal's units aside from their units scouting the map. Attacking any other unit will lead to the AI turning aggro.

Once the Castle is completed, use your Longbows to start sniping Teal's units. This will turn his entire army upon you. Make sure to lure all units within castle fire. While you're doing this, use your Hero Treb to shoot at the buildings around Wallace. As a note: The treb has higher range than Line of Sight and will unpack if it can't see a building, but won't attack.
Once the trickle of Teal units has slowed down, start sending your longbows in to grab Blue's units. Kite them to the range of the castle. Blue has 3 Siege Orangers. Wallace will not wander too far from his hill and will dodge longbow shots. He requires melee engagement to fight.
Aymer also makes a good kiter due to his speed

Once the 3 Siege Orangers are dead and Teal isn't sending a massive number of units, you can send all your units in to kill Wallace. Once Wallace is dead, Blue and Teal will both resign and Robert will immediately turn on you. You'll unlock the achievement once this happens.