If you struggle in the last two waves of Arena Challenges see this video, stay in this corner and get your achievement!
This video show a easy way to you get the achievement (but takes a little more time than usual way):
Credits to TheBrothersGrim666 for make the video.
Another tip is: make sure that your difficulty (in Options menu) is set to Easy, to obviously make things more easier.
Hope that helps, if you see anything that I can improve in this solution please send me a PM or comment below. Thanks.