OK, so this is driving me crazy. Been going for this for about a week now and I can’t find a guide online about this particular achievement.
“Foil a Hunter’s ambush twice while allowing it to Flicker both times, then kill the Hunter.”
On Wave 21 I have managed to let it Flicker twice. It’s about 24-26 seconds before it comes back to stun with Ambush. I have dropped CF on myself and activated Purification drone, seemingly blocking the ambush from happening, then killed the hunter and no achievement.
Does anyone actually know how this one is done?
Purification drone won’t prevent the ambush, it’ll just remove the stun once it’s been applied. Try a Phoenician Support Stratagem (I think that’s what it’s called) which prevents crowd control from being applied at all. Also make sure you prevent two ambushes, not just the second one.
If that doesn’t work, I’m guessing you’ll need to stun the hunter before it finishes casting ambush. 
Thanks! I will try that gadget in my next few attempts.
Have you tried impairing it’s column attack - letting him flicker - impairing column attack when he returns - allow him to flicker for second time - then killing when he returns the final time. There may be some confusion with what ambush is refering to since I got the achievement without any cc immunity. Or I just was very lucky.
When it returns after flicker the ability it casts to stun you is actually called “Ambush”, I believe.
Have you tried impairing it’s column attack - letting him flicker - impairing column attack when he returns - allow him to flicker for second time - then killing when he returns the final time. There may be some confusion with what ambush is refering to since I got the achievement without any cc immunity. Or I just was very lucky.
That crow attack I remember what you’re talking about. I haven’t tried that since the achievement refers to his ambush attack he does right after he returns from his Flicker.
When it returns after flicker the ability it casts to stun you is actually called “Ambush”, I believe.
You’re correct.