i think the "hardest" boss in the deathsands is the golden dragon and you will loose if you use maria because you will not be able to generate enough damage with her. for the quests the hardest boss to do is most likely the spirit of the dragon king in the searing sands. but he is super hard with or without maria because he is bugged and you wont be able to attack until you cleared the top row, but have to do it in sync with all other minions or the subs will spawn in the top row and blow any chance on killing the boss.
so again, i stick with my opinion that you do not have to have maria - i agree on her making it a lot easier though.