I'm not sure if this is considered as an exploit or if it's even THAT profitable.
I never liked farming champ boxes, but if you get a party of 5 people, here's what you can do.
Finish HotW story mode. At the end of the story there are champion sharks in the water (which you can hit from outside if you like to avoid combat) which also respawn in about 10-20 seconds after you kill them. And yes, they drop champ boxes