I Didn't Hear No Bell

When you defeat a prey after an ambush there is a projectile flies out from your character towards a random location, there will be red smoke on the ground where the projectile stops. Walking into the smoke reveals the prey and allows you to riposte. The prey will ambush you about every 3-4 minutes whenever you are in combat or when you disarm a trap.

If you are somewhere which does not allow the projectile to travel anywhere, like inside a small cave, then the prey will land pretty much on top of you.

Here's how I did it, with wisdom gleaned from other comments and some of my own.

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• You can get the prey to ambush about once every three minutes. This is best done in a secluded area without interference.
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• I did this while progressing the Prey: Normal Made III achievement.
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• After the prey goes immune to damage, look for the red projectile, which points to the red cloud.
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• As you approach the red cloud to deliver the riposte, start spam clicking the area where the prey will appear. Sometimes you can get two credits towards the 50 you need.
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• UPDATE: several other wonderful posters have pointed out, using the dummies in Voidstorm works great! /way 40, 81.5
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• Here's a list of toys you can use to stay in combat to trigger the prey: Sturdy Love Fool, Turnip Punching Bag, Rubbery Fish Head, Tuskarr Training Dummy, Black Dragon's Challenge Dummy, Ancient Construct, Barrel of Fireworks
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• Here are some good spots for putting down a punching bag toy:
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• Zul'Aman: /way 32.7 22.4 Zul'Aman Riposte Area
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• Harandar: /way 52 39.2 Harandar Riposte Area
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• Voidstorm: /way 53.2 59.1 Voidstorm Riposte Area
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This is the best way I've found to do this. Fight the training dummies at 40,81.5 in Voidstorm when you're doing a Hard or Normal prey. Every 3 minutes, you'll get an ambush, and it's easy to watch the red smoke up there. And nobody will bother you!