Total War: THREE KINGDOMS

To ambush, set your army to the same-named stance. It requires 25% movement points and will hide your army while preventing it from moving or doing anything else. The base rate of success I think is to be 45%. You can boost your chances in certain terrain (hover any spot on the map with an army selected to see your chances), as well as with certain skills and ancillaries. If I remember correctly, strategists and champions have access to the ambush-chance increasing skill, and is also applied to the army they're attached to instead of only being applied when the character is the commanding officer. Most AI armies will follow paths leading to your cities or counties, but you can also try to goad enemy armies into an ambush by putting another weaker army right on the other side of it.

If I remember correctly, the achievement only unlocks after fighting the ambush battle. It doesn't work with an auto-resolve. You should almost always have a significant advantage as the ambusher, but if you want to know more about them, ambush battles are conducted differently from normal battles. The ambushed army will always be spread out in a long line, and you'll be able to deploy your forces anywhere just outside of their path (you'll also usually have forest cover in your deployment area, but I don't think I've done an ambush in a more deserty location before. The enemy is always open on something like a road). Keep them as scattered as possible and try to fully isolate units when possible. Use champions or sentinels to keep the enemy generals busy (especially vanguards). I think the ambushed gets a debuff to their morale. If true, take advantage of that and force units to rout before moving on to the next one. Chaos is your friend here, keep it up as long as possible.