I was playing a tournament and I matched a lvl 100 that had defeated kChamp, I defeated him and I didn't get the weapon or exp I should be getting from completing the challenge...
You can now beat either the developer, or a person at level 100 who already has Godrays to get the achievement, best wishes in your endeavors! ~
And that's what he/she did, beat lvl 100 but it was in a tournament and the achievement was not received.