I did some testing last Friday. The test are a bit inconclusive and I am planing to do some more. The test involved bringing a player up to 1st powered down and then doing several things like being attacked and attacking and wining and loosing. The equipment and runes were not change so the power levels remained the same. The position of the hero on the grid was changed each time. From this limited testing, on a hero player that coming up to 1st or 2nd it looks like the formation is the last battle that is won, and not just the last battle.

Next I am going to do some testing where the power level changes to see if that affects the result.