The problem from what I understand is not the difficulty of the achievement, but the many clueless players that ramdomly join 4p rPVE10 lobbies without proper knowledge of what to expect and how to win a rPVE10 map. These people are not always carryable as rPVE10 is hard (as it should be). So these folks are massively diminishing the enjoyment of regular rPVE10 players like Volin and the duck. Hence the suggestion to remove the achievement and thereby removing the incentive for players to join rPVE10 without the proper preparation.
Switching the achievement to singleplayer would prevent frustration and lack of enjoyment of the game for the regulars.