This achievement text reads:
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Guard Breaker (10 Points)
This special challenge can be completed during the mission "Fall of the Tae Ew"
- Defeat all the Amygdalan Guardians within 4 minutes.
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In order to complete it you must kill the all of the Amygdalan guardians, within 4 minutes of killing the first.
The easiest way to do this is to kill all the Tae Ew, leaving only the guardians up, on each pyramid; then kill each guardian in quick succession.
There is no item, xp or faction reward, just the 10 achievement points.