Does it matter which difficulty? I was able to beat Legend with a max level Luci helper with a modest setup:
Dark starter as lead, 2 Awoken Heras, max skill vamp, and RK
Everything went fairly smoothly. Double gravity + luci skill can be used to quickly take out one of the demons (assuming you have heart orbs to heal back). To kill valk I took her down to half hp first and then used everything I had. I would have ideally brought a heart changer if I had hp to spare, but I wanted to make sure I could tank valk's hits even if luci got bound.