The Waylayer achievement requires you to win 5 ambush battles.
I am 100% sure I won over 10 with Skaven by playing and winning mannually in the Eye For The Vortex Campaign. And 6 with High Elves in the Mortal Empires campaign (I did not autoresolve). In all cases as the attacker.
This did not give me the achievement.
I suspect the achievement text is wrong, as the achievement itself is not locked or bugged, because around 1,6% of players has it. So maybe the achievement is about winning 5 defensive ambush battles, or winning with a certain race.
I hope someone can give more clarity and CA can be more precise about the requirements.
Thanks for a great new game and the free new merged campaign.