Go into the online mode my set is as follows I chose the same characters for defense as offense . Darksied level 9 , Scarecrow level 7 , and Harely level 1. Make sure you equip the best gear for each fighter and you'll get this with maybe 3 loses fairly quick . The mode is strangely addicting as you do gain xp and guild XP.
Also I searched for the teams with the lowest hero levels so their wouldent be a factor. I just to mention this can be repeated daily for an easy 5 gold boxes daily.