Allo there,
It doesn't have to be a hand cannon to kill the invader. I did it with Truth when I did mine. It has to be within the first 10 seconds of the enemy spawning on your side, with no team mates killed.
English
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As stated to TuxyDoh, no teammates downed and used Chaos Reach super to down the invader, earned the 'Not on my Watch' medal, quest did not progress from that point.
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Unfortunately you'll have to try again. If you can grab a video of it happening that would be great.