The challenge for Arena 2.3 is to "Kill an enemy champion on your first turn and win a battle."

I've done this multiple times, but the challenge won't complete. I saw an old forum post that stated there was a bug with the Shield Set so I made sure none of my champions had that set and tried again. Still no dice.

That old forum post was 2 years old and this is still a bug. That's very disheartening for a newer player like myself.