Next time you're attempting this, if you could get a video of the attempt, that would be very helpful (and give me something to submit to Plarium, if necessary). Thank you!

im stil having the same problem and i cant attach this video to my support request for some reason so I thought maybe i could gett some help posting it here at least

Video 

Hi, I have the same problem, I also have tried everything. I contacted support and they told me I have to kill ALL THE ENEMIES in the first turn, and than makes no sense because the description is "kill an enemy champion". Anyway, I have tried killing all in the first turn, killing just 1 in the first, using 4 champions, using just Ronda, everything! The worst part is that support team does not admit there is a bug. Or maybe is the most difficutl challenge of all times?

Hello, I have the same problem, and I've tried everything. 4v1 instakill, 4v4 istakill, 4v4 only kill one and then win, nothing seems to complete this quest. I tried extracting some video, but my instant replay got fucked so I couldn't. I'll try make some videos later, if that helps. There's nothing I can do to complete this quest.

Having the same problem. Had this challenge for nearly 2 weeks now and with Rhonda always going first making a variety of 1-4 kills on the very first turn, doing 5-15 battles a day. How has this not completed yet?  

Rhonda, with the speed gear I put on her, goes first 97% of the time and kills 1 or more enemy champions 95% of the time on first go. 

I've tried single enemy hit rather than an AoE right out if the gate, and that still didn't trigger the challenge to complete.

From the information given, I'd say the challenge is bugged. Otherwise the challenge text needs changed to the correct conditions that need met in order to complete it.

If they really said you have to kill "ALL THE ENEMIES" then the challenge text is wrong. I too have killed all enemies on the very first turn, and even that didn't complete it.