Large loft area cooling
You need to raise the returns to near the ceiling and possibly add more supplies and returns. A picture of the return and supply registers would help determine just how hard it will be to raise the  returns. Also, can this room be isolated from the lower level or is just an open stairway. In some of the condos near where I live it is not possible to maintain a comfortable temperature in these upper rooms without a 2nd dedicated unit due to the homes construction.