This achievement is a bit tricky as the description of the achievement isn't 100% correct.
You'll need to win 5 ambush battles, but not just during a single campaign, but during a single session. Saving and quitting the game resets the progress of this achievement! The other big thing is you can't auto-resolve the battles, they don't count. You need to play the battles manually!
When you want to go for this achievement, make sure you have some time to play as it might take 30 minutes or more depending on what faction you play, what armies you have and what state the world is in (aka which enemies live etc, how many armies they have).
I personally got this by starting a Deathmaster Snikch (Clan Eshin) campaign on the Vortex campaign, but Mortal Empires should work fine too. As Skaven, the regular stance gives you a chance to ambush enemy armies when attacking. You can of course also use the ambush stance (which requires 25% of movement speed) but it's pretty easy, as long as there are enemy armies walking about outside of settlements. If you start Snikch like me, one simple way is to farm on Sudenburg. Attack his armies, if it isn't an ambush, retreat. Keep repeating over the turns with your army/a second army you recruit until you've done 5.
It might even take 6 or 7 won battles, I think I had to do 6 before it unlocked, so please keep this in mind when it doesn't immediately pop after the 5th.
tldr:
5 ambush battles, during a single session (no quitting the game or going to main menu beforehand), play them manually, might take more than 5 to unlock (like 6 or 7).
If anyone has extra questions, or wants me to clarify a part more, lemme know :)