the achievement is actually called "number won hero," but due to a visual bug, it appears as "Team Balance." Number won hero is achieved when you defeat the demon king at level one.
edit: I was wrong, I recieved the number won hero achievement before the Team balance achievement. When I did, the "Team balance" achievement did not appear on the screen as an available achievement once I got Number Won Hero. I thought since it disappeared that it was completed as Number Won hero since I didn't see number won hero anymore.