This achievement is fairly easy and common for people who are on higher difficulties or aren't taking time to explore but those on lower difficulties and/or who take the time to fully explore and level up their archetypes as is required for another achievement might find it difficult due to the fact that if you're a higher level you cannot stun enemies.
To get it under normal circumstances all you have to do is attack enemies of a similar level or higher in dungeons with X then press Y when the Squad prompt comes up. You will do ambush damage through this and after doing it 100 times you will unlock this achievement.
If you find yourself stuck with this one due to being overleveled there's a trick you can use in the final dungeon detailed in the video below to grind out your remaining ambushes. This won't work if you're max level (or close) since the enemy it uses is around level 88 but unless you've truly gone to excessive leveling this shouldn't be a problem: