Age of Empires IV: Anniversary Edition

This achievement can be unlocked in the first mission of the Hundreds Year War campaign. I did this on the easy difficulty and only had 20/30 alive so if you're struggling you may have to attempt this on the story difficulty.

To begin proceed through the mission gathering champions and doing their tasks. Towards the end of the mission, you will have to select which 2 champions you would like to compete in the arena. Before you choose make sure to save just in case you lose more than 10 units or if you want to try different champions. Loading a save may cause your achievement to not unlock. So if you are attempting this achievement try it on the story difficulty and complete it in one go instead of loading it back if you lose more than 10 men.

I chose the champion with the spearmen and the champion with the cavalry. The spearmen's champion has a healing ability which comes in handy and cavalry are great against infantry who carry bows and swords. Line your spearmen up and wait for the enemy to charge at you first. After getting engaged use your cavalry to swoop behind and thin the ranks of the enemy. Midway into the fight, the round will end and you will have to take a small break by buying upgrades. Make sure you heal before heading back for the final round. Repeat the tactics and finish off the enemy.

If you still have at least 20 units surviving the achievement will unlock. If not you can load it back to before the fight starts and try again.

To get this qick, choose "story difficulty" and start the mission from the main menue. For me changing the difficulty while already in the mission (therefor forcing me to restart it) did not work/pop the achievment.

The first part of the mission is pretty simple. Use your units in a favoring rock-paper-scissor pattern (speerman vs cav.; cav. vs archers; man-at-arms vs speerman). Use your hero units if they are not to low on healt and dont forget to use there talents/powers.

For the final fight the AI uses speerman and men-at-arms. You can easily get an advantage if you choose the hero related to healing - and he is a must have. For the second squad Ive chosen the moral breaking hero, since he debuffes the enemys.

In the fight you just have to split yout troops in two long lines, creating a wide V shape that traps the englishmen. Idealy the men at arms are at the center of the formation to tank the incoming dmg, but it should work out any way.
Once the enemy is in the center of your trap you should use a attack move order and engage (a+left mouseclick). As soon as your troops surround the enemy use the moral break ability and your main heros attack speed buff.
Once some of your units reach half health, use heal. If its just a singular unit that drops below 50% health, you can just pull him back, since the AI tends to focus on the next soldier in line instead of chasing fleeing folk. Only real thing to point out here is that some units block each other from fleeing. Just select the low health one with 2-3 others around him and move them back a few meters, then single out the weak one and move him back further. The others sould join the fight again on their own, but you can always give the order.

Once the first round is over, just get back outside and let your health and abilitys recover.

Get back into the arena and repeat. Since you should have a huge numbers advantage this should be much easier. With this tactic I only lost 2 troops due to not looking, I believe that a 30/30 population can easliy achieved this way.

You need to complete the mission, so make shure to get/skip the cutscene.

Notes: Im unsure if the composition of AIs forces is randomised/difficulty depending. I recall that I had to fight archers once.
Achievment pop might take a few minutes. (Just start the mission a few times over and head back to menue might help, but this is pure speculation)