Once the battle starts, just run around the corners of the map, let the enemy chase you and once they are close, move to another corner, you can speed up the battle, and once the time is over, you´ll win the battle and the game will give you the achievement, because your general surviving the fight, counts as winning ;) .

You can also try to hide right away during the deploy phase, the enemies will look for you, with some luck they won´t find you but be prepared to start running if they do. If you hide again after they see you for the first time, they´ll be able to track your new location rather quickly, so be on the move for the most part.

The battle where I got it:

https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=3314118839

After one of my main armies defeated one of the armies of the Normands, this particular army retreated to a little town. So I took the chance (since their cavalry was killed in the previous battle), to bait them into attacking my lonely general that was besieging them. It worked on the first try.

Note: You might need to pass the turn for the game to give you the achievement.

This is my first guide. After so many tries, I found this easy way to get the achievement almost by accident. I´m not 100% sure, but I think this little "trick" can also work for the "Legendary Defender" achievement since if you farm this process, your general will easily have multiple heroic victories while defending.

Some screenshots of my general, hiding during the battle (before having to run around the map).

If you notice that in the screenshot of my general, where he is hiding he has a 69 unit, and in the other one he has 66, the reason is because the first time I tried, when I got the achievement.

I didn´t even try to hide and just ran around the map, after that since my general didn´t kill the enemies, they attacked him again in the next turn and this time my general had level up, so that´s why.

In the second "battle" I tried to see for how long my general was able to hide